Loud Noise
 

I have a strange problem that no one has been able to solve. I provided as much information as I think I could, but feel free to ask for more. It's a very peculiar situation!

The car is a 1994 Pontiac Grand Am, V4, and it has been making a terribly loud screech, almost sounding like metal scraping/banging on metal. It also sounds similar to the noise made when one turns on the car, forgets that it is on, and attempts to start the car again. It is painfully loud, obnoxious and annoying.

The noise randomly occurs and it is usually unexpected, however long trips and hard (heavy acceleration, high speeds and a warm climate) generally are able to make the noise come out. However, short and easy trips can do the same thing.

The noise occurs after the car has ran for really any amount of time and does NOT occur while the car is in drive or reverse. Just when it is in park or sometimes in neutral (it can be fiddled with to let the car be in neutral, but usually slips and starts the noise again). The car usually affects the lights (head, tail, dash and cd deck) by causing them to slightly dim and vaguely flash in cadence with the noise's rhythm.

When the car is turned off, the sound disappears, but if it is started back up soon, it is sure to come back. Once the car was on and in park and the noise began after about 1 minute of running.

Sometimes when the car is put in park and turned off, but the ignition is still at the "on" position, the noise will occur, but usually more slowly and 'pronounced'. During this stage the lights flicker and strobe in sync with the grinding/banging's rhythm. I applied the brake at this stage and it seemed to slow the frequency and intensity of the noise down while the brake was depressed, and would return to normal while released.

The car has been to a mechanic that checked the pulley's and could not physically see anything that would cause the noise. However, it was not currently making the sound.

A trusted family mechanic thought that it might be the transmission and had the fluid drained and the filter replaced. He could not find any excessive metal shavings or anything to lead him to think it was the transmission. He replaced all the motor mounts (since they were badly worn) in case it was the engine knocking (which fixed another problem).

We have recently been able to turn the car off while it is in drive and this eliminates the noise from occurring. I like this feature, but it doesn't happen all the time. I don't think this is a normal thing to happen either. Could this be a problem with the starter?

Based on the information, does anyone have any experience with a similar problem and know a solution?