99Accord V6

When I park this car after driving home from work, in about 30 minutes
it starts making an intermittant bleating sound. Somewhere between a
fart and the sound of a sheep. Seems to be coming from underneath the
back seat driver's side. It usually goes away after an hour or so. The
noise seems to be getting louder. Any ideas on what might be the
cause?

 

You might want to make sure there is not a sheep or large dog in your
trunk--just kidding.
You did not mention whether the car is on idle or the engine is not
running when it makes the sound. Regardless, if it happens after the
engine is off, crawl under the car and see if you can determine exactly
where the sound is coming from. If it happens when the car is on
idle--make sure it's in park and the emergency brake is on when you crawl
under the car.
Another option is to take it for service and have them put it on a lift.
It will be much easier to identify the source of the sound if the car is
on a lift.