1999 Pontiac Grand Am new noise. Power loss

I've got a new noise coming from my engine. I have done all of the work on my car, but can't figure this one out. It is cold out right now, so I haven't exactly investigated yet, I'm just looking for somewhere to start. This came up 2 days ago. When the car is stopped and idling I hear almost a "vacuuming" type of sound coming from the lower rear of the engine block. Today I noticed that I am also losing power when I stop, my headlights and interior lights dim until I start moving. The noise is there until I hit about 10mph at which point it is either gone or has been overpowered by road noise. I'm just looking for thoughts on where to start. The car is a 1999 grand am. About 3 months ago I did a tune up and replaced the spark plug coil pack due to some ignition problems.
any ideas on where to start? Is it possibly something in the exhaust? Any help would be great.