Check engine light, diagnostic vvti sensor, camshaft

Sounds like a bad rod bearing, no cure except a rebuild. I assume you do know the difference between spark knock and bearing knock.

Does your vehicle have a serpentine belt ( flat and wide, ribbed on the inside)? if so,first I would check the belt tensioner. You'll need a tensioner tool ( rentable from Autozone with a refundable deposit). Before starting, observe the routing of the belt, as this is crucial to your motor ! Release the tension on the belt and slide off of the tensioner. Holding the belt out of the way, spin the tensioner. listening for the popping or snapping sound. If the tensioner is ok, check the idler pulley the same way - I'm pretty sure it will be one of the two. Other than this, I would need more specifics of your vehicle in order to research. If the motor is not running hot, I would not suspect the water pump - which may or may not be operated by the serpentine belt.I would not run the motor until you determine the problem - most newer motors are interference motors, meaning the pistons cannot be in the up position while the valves are open, or else serious damage will occur to the valves ( typically the exhaust valves and guides are severely damaged).Also, if it is the tensioner or idle pulley, you could lose the belt while driving which would knock out your power steering, alternator, and possibly your water pump.
I hope this helps and thanks for using Fixya!!

If this vehicle was driven without oil, the rattle or knock you hear is more than likely a damaged main bearing or rod bearing...sounds like its time for a QUALIFIED PROFESSIONAL to listen to your engine run and give you his opinion... Good Luck!!!

that would be the engine front , you sure it's a Rattle ? if so ? it will be your Camshaft bearing noise, is it a 2.5 L 4 cylinders ?or 3.5 L V 6 !! .
if it's 2.5 L , your Camshaft bearings are starving for oil , so I suggest you check them both intake and exhaust cams and see also if the valve lifters or cam bearings are in bad shape , worn out ? check also both cams journals and drive journals to lifters ( contact operation ) if there is a suspicious gaps between them that also can cause a rattle sound .

It does sound like one or more of your big end bearings has failed. Your valve clearances are more of a light sounding rattle, and don't usually give trouble unless the engine has become sludged. Camrys can suffer sludging if oil changes are neglected.

To get some idea what is wrong, have a mechanic remove the oil pressure sensor and insert an oil pressure gauge. You should see (when hot) about 10 psi per 1000 RPM or so. If this is down, you have bottom end bearing trouble.

READ THIS (happy ending so far...):
audi a4 2L turbo quattro, manual transmission with 98,000 KM. Perfect conditions, no oil leaks whatsoever.

Left the car off for two weeks in very cold weather (-10C down to -25C). When I restarted, it started immediately but with heavy rattling noise due to camshaft adjuster not getting enough oil pressure. After 3 minutes of warming up (outside temp.: -12C) at very low rpm (below 2,000rpm), I start moving the car. Still hearing heavy rattle noise. As soon as I pass 2,000 rpm, I get a scary red warning, low oil pressure. Turn off the engine immediately. In 1 minute, I re-start the car and the engine lights check ok, so I drive in first gear super slow (1,200rpm) hearing the same rattling noise. After another minute, finally the noise all the sudden disappears and the car starts running just fine.

Called Audi dealer and they told me that in extreme weather conditions, I should warm the engine for 10-15 minutes if necessary before start moving.
Take away: luckily, I didn't screw up the engine since I was using it at extremely low rpm (easy to do with clutch and manual transmission) until the oil reached full pressure. I am pretty sure now that the problem was leaving the car off for such a long time in freezing weather. All the oil flows back to the bottom, and when I restarted it, the cold oil with high viscosity took a long time to reach all the parts of the engine, specially the camshaft adjusters that are actuated by using oil pressure. Next day, the car started at -9C perfectly without making any noise (as usual). Basically, the longer you leave your audi in cold weather, the longer you might hear the rattling noise until the oil reach full pressure inside the engine. My car took 6 minutes to fix it after couple of weeks in cold weather. This makes sense to me; in fact, in the past I only heard the rattling noise after leaving the car off for a couple of days in the winter.
So far, this has had an happy ending....

Man what a hard time you have had!
Before you can trouble shoot the possible bearing problem,your engine would need to be able to run.

Your engine will run with a bad alternator for awhile with a fully charged battery. This will allow you to have someone listen to the engine and evaluate the bearing nose.

A bearing rattle when you rev the engine up is usually a rod bearing.
A bearing rattle when you declerate is usually a main bearing.

I hope this has been some help.

It sounds like you might need a mechanic help you make some decisions

Good luck

If it is a 'squealing' noise it's probably the alternator/power steering drive belt. Before you turn on the engine in the morning, lightlt smear or spray some oil on the inside of the drive belt to lubricate it. If the squealing stops/disappears, then that's the fault ..

That squealing noise can be caused by an incorrectly tensioned belt, or a worn belt. Replacement is the only option.

The other frightening sounds can best be described as 'rattles' or 'tapping'. When a car stands overnight all the oil drains back into the sump (oil/pan).

There are two things (usually) that can be cause the rattle/tap:
1. Because the oil hasn't been pumped around the system, at start up worn big end bearings can rattle and tap on the crankshaft until the oil 'fills the gaps'. The tapping/rattling goes away as the engine warms and oil is pumped around the lubricating system.

A 'bottom end tapping' is a symptom of worn big ends (they are bearings at the bottom of the piston and fit around the crankshaft).

2. A rattle/tapping from the top of the engine can either be a worn camshaft or 'lazy tappets'. Again, when the engine warms and oil gets to the camshaft and tappets (also called 'hydraulic lifters) the noise will disappear.

If your car has this rattling/tapping noise - identify whether it is coming from just under the rocker box (ie the camshaft/tappets..) or the bottom end of the engine (big end bearings).

If the noise is coming from the top of the engine, try an oil change with a can of thick 'additive' included. A lazy tappet (lifter) isn't too much to worry about. A worn camshaft will affect the performance of the engine, though continue to do its job. An oil change with an additive included may help in quitening things down.

A bottom end tap/rattle (or 'knock') - the sooner that is sorted out the better.

swap the asd relay with the fuel pump relay and see what it does then

If this sounds like a tin can rattle it is likely a loose heat shield on top of the catalytic converter. They come loose after time and corrosion take their toll.