Hi,
I just bought a 98 Deville with the 4.6 Northstar. I can hear a fluttering sound when accelerating while the drivers window is down. The sound seems to be coming from the left rear somewhere. It seems to dissipate when I let off the gas. Any ideas as to where to start?

 

The only way you can pinpoint this is to get out of the car and have someone else accelerate the engine in park and see what you hear. If nothing then I suggest having the person put it in drive and hold the brakes and accelerate so as to put a load on the engine and listen again. You have dual mufflers in the rear one on each side so that's what I'm thinking of, flapping baffles or leaks?