I have a very loud rattling sound coming from my 99 Jimmy truck When I hit the gas petal the sound becomes louder but only for a sec and it goes away Every time i hit the gas petal the very loud rattling sounds comes When i am in park I can hear that sound only a little When I do hit the gas petal hard it sounds like it is shooting fire but after a few secs that sound goes away The sound comes and goes while I am driving but still loud Could this be my muffler

SOURCE: Rattling post 2000 rpm in every gear

Sounds like the timing belt tensioner. I have that problem. Rattles at low rpm, goes away as you acceralerate. Looking to have timing belts changed and tensioners replaced.

SOURCE: LOUD RACE CAR NOISES FROM MY 99 VW BEETLE

This seems to be a classic case of "disentegrating exhaust system syndrome." There is either a hole(s) somewhere in your exhaust system or there is a leak somewhere. Probably the easiest way to tell where the noise is coming from is to start your car and just listen around in the engine bay, under the car, and at the muffler. Also do a visual inspection of the exhaust system to see if you can see any holes in the piping. I have also seen hairline cracks in the exhaust header itself that can cause the loud ticking noise that can be a little tricky to track down.

SOURCE: 2004 Ford Focus ZTS engine whine/ noise

The exhaust system on this vehicle has to be able to flex or it would crack at one of the joints, so it has hangers with rubber bushings to hold it into place and still be able to flex with the engine. Now grant it one of these bushings may have come loose or one of the hangers may have broke but I would be concerned about the thumping of the exhaust against the floor board. This could be caused by a broken motor mount letting the engine flex to much. I would suggest having someone inspect these.

SOURCE: rattle noise from engine at start up for a few

The oil pump is not getting oil into the top of the engine quick enough on start up

I have a similar problem with a Mazda

Got over it by adding a can of STP Oil treatment to the engine

That coats the surfaces better than the standard oils and cuts down on the noise and the wear factor

SOURCE: relaced transmission and now have a rattle sound

your torque converter is rattling that is the clutch on the stator inside is worn out... You can stall test your torque converter ...Do this but follow correctly. put on parking brake..on the asphalt,,,, block all four wheels...push the gas to the floor for just 2 seconds...down one, two lift off gas pedal quickly..
when your doing this WATCH your RPMs they should reach to around 2200-2400 between the 2-4 rpm if not the Torque Converter is toast!!!! If your car passes the STAll test then when you tranny was put back on one of the fly wheel covers are rubbing....!!!.