Loud buzzing noise after cold start. 93` M5

[NH004]

When I start my car after it has been sitting for a couple of
days, there is a loud buzzing sound that occurs. I can`t
quite locate the exact source of the noise though.
It stops after about 15-20 seconds. or if give it a little gas.
The idle is fine, and it doesn`t seem to be effecting the
cars performance in any way... It`s more annoying than
anything. anybody got some input?
Thanx.
NH.

[TheGeak]

Is it the o2 injector? If you didn't know the M5's have an air pump that pumps oxygen into the exhaust system on cold starts to keep the emissions down. Perhaps thats what you hear?

[NH004]

sounds about right. thnx.. :D

[Jr ///M5]

The purpose of the air pump is to help flush the richened fuel mixture at startup out of the cat. The unburned fuel that sits in the cat will melt it when it finally ignites. The air pump simply helps purge any unburned fuel out of the cat until the engine is out of the enrichened cycle.
The sound of the air pump is more like a muffled pump sound instead of a hiss or buzz.
Check the rear of the car by the gas tank area, chances are, it's the fuel pump you are hearing. You might try listening very closely with the door open, turn the key to "on" position, but don't start it. You should be able to hear the "buzz" for just a second or two, see if it's the same sound you hear when it's running.

Hope this helps,
Jr