My car makes a rattling/vibration noise - under the Hood when I am sitting idle at a light or parked (while running) - Only notice it happening when the Car cooling fan kicks on and my Air conditioner is on. What is this noise? loutd noise not the standard noise - I have 154,000 Miles that I have placed on my car.

Catalytic converter substrate may be making the rattling . Does it do it at idle in park ?

Exhaust system loose

The technique I always used to identify where such a noise was coming from was to use a very long screwdriver (2 feet long), while the engine is running. CAREFULLY touch the screwdriver end to each pulley or other object under the hood, and put your ear to the handle. The device making the noise will become very clear.

It could be AC pump/clutch, alternator, water pump, power steering pump, etc.

Chances are that a heat shield located under your exhaust or the catalytic converter has broke loose and rattles when you are idling in drive. This is common on older cars and is only usually noticed while idling in drive because the engine is working under a slight load and runs a little rougher. A good exhaust shop could probably fix it for very little.

I have an 04 zts and mine used to make a lot of noises like you have and I had a bad motor mount on passenger side.Replaced it and no more rattles like that

It's your exhaust mounts or broken exhaust bolts. It does it in drive st idle n under load because the motor is sitting in rubber mounts made to dampen vibration. Therefore the motor tilts to the left in reverse pulling the exhaust away from your under carraige, and to the right in drive causing the loose exhaust to vibrate against something. Check your heat shields also. They fall apart n do the same noise. Piece of cake.

Check to see if one of your brackets holding the exhaust pipe has come loose. That'll make a rattling sound - especially if it towards the end by the tailpipe. When you're accelerating - more air is forced thru your exhaust at a faster rate, so it's not moving as much as when you're sitting still and the truck is just at an idle with less air/exhaust moving thru the tailpipe. If it's only making the noise while the truck is stationary & the noise stops one the truck is moving - that pretty much rules out anything to do withe the drive-train or suspension. You are sure the noise is from the back & not coming from under the hood? If under the hood - check your oil level ASAP.

open the hood have someone u trust sit in the car and put it in drive holding the break with it in gear tap the throttle quick if the engine looks like it wants to jump out its a mount issue granted it will move about 3-4 inches if there ok anything more you have a problem try it in both (D) and (R)

while driving at higher speeds you may not notice a sound as much as when you are drivng in reverse as you usually drive slower in reverse. This leads me to think that the rattling can be caused by a loose protective plate under the car. When a load is put on the engine, it tends to vibrate more this vibration can cause a lose plate to rattle more. This is the cheap answer. That is if you are definitely sure the sound is coming from under the vehicle. Otherwise, you may have a crank shaft problem or a seal in the bottom half of the motor could be bad. You may want to crawl under the car and try to shake some of those plates underneath. If none of them are loose take the car to BMW it shouldn't cast anything for them to look at it unless they have to hook it up to the computer.
Please remember that if you intend to climb crawl under your car, you must make sure that your car is properly supported to prevent serious injury. Also, if your driving with a loose plate, it could easily fly off while driving or if you hit a bump. This could cause serious damage to your vehicle. Also, please check you oil levels and make sure you do not have an oil pressure light coming on when you hear this noise.

Hi and welcome to FixYa. Please try this:
GO out to the car, and lift up on the hood. Does it raise a bit and rattle? If so;
Open the hood, and look at the perimeter of the front radiator support. You will see 2 small rubber thingys, that look like that they are screwed onto the metal that is holding the radiator. Unscrew them a little, and close the hood. Pull up on the hood and see if the rattle goes away. If not, repeat unscrewing the rubber things until the lift disappears.

A FixYa rating (4 blue diamonds) would be greatly appreciated for my assistance.