My 1998 Expedition (Triton 5.4, 2WD, automatic transmission) often (about 95 percent of the time) makes a strange growling/vibration noise when it is in park or in drive and stopped at a light but runs great otherwise. If I put it into neutral, the noise stops completely, and as far as I can tell it makes no noise when it is in gear and moving. On rare occasions it doesn't make the noise at all when I am stopped at a light, which is usually when the noise is loudest. I have had a couple of mechanics look at it. One said he didn't think it was the transmission and nothing to worry about. Another one said that the harmonic balancer was loose (it was) and replaced it, said that might take care of it but that did not get rid of the noise. Any ideas out there??

 

It could be broken blades inside the torque converter.

 

It sounds like a vibrating head shield or piece of sheet metal vibrating away. My pickup does it once in a while. Nothing to worry about.

 

The Symptoms of a Bad Torque Converter | eHow.com

this article says the cause is faulty needle bearings in the torque converter.

other articles on this site say similar things.

 

Your issue sounds like the problem I had with my 2000 Expedition. The Air Compressor for the air ride suspension would go off even when the vehicle wasn't running. It was loud because the rubber grommets that attach it to bracket had worn off and it was metal on metal vibration that would make a growling sound. THe pump is located on the passenger side under the wiper fluid. I obviously had a slow leak which is what made the compressor come on periodically. I just replaced the air ride suspension with the Monroe conversion kit and shut that air ride compressor off.