Sad exhaust tone from my 4.4i out of the left exhaust?

[Estroil 4.8is]

I had my brother take my 2000 4.4i for a spin while I stood on the sidewalk. I noticed that when he gunned it the driver side exhaust sounded like sh!t. The engine doesn't misfire or skip during idle or excelleration. It does have a slight hesitation when reving the engine at a stand still. It kinda sounds like it under water or that the air is having a hard to escaping the exhaust. Definitely does not sound like a V8 . I thought about changing the plugs but the X only has 69k and the engine doesn't skip or anything out of the ordinary. Maybe a vaccum leak? Are there any known problem area to check for?

Thanks for any insight or ideas, Rob