Possible G6 issues

Hi all:

New member here. I'm a previous driver 2006 G6 base sedan and when the lease came due this past November, I bought a used G6 GTP coupe from the same dealer. I'm pretty much up to speed on G6 issues...came across the intermediate steering shaft problem (replaced under warranty) and the occasional clunk from the steering column. The sedan I had was great aside from that.

The GTP is miles beyond what the sedan was and I'm enjoying it. That being said, I may be having an issue with my new ride. I'm thinking I have a bearing or ball joint problem but not sure. I can hear a hum coming from the car between 40mph - 60mph. It goes away after that, and only does it on the freeway. Turning the wheel one way or the other does not make a difference, same pitch. It is weird because it only does this on certain sections of the freeway, but if I travel the exact same speed on other roads, I don't hear a thing. The previous car didn't make this sound on the same stretch of highway so I don't believe it is the road.

Just a few quirks here and there...The car also shakes periodically at cruising speeds...more noticeable in the cold. Also I notice that there is a constant 'hiccup' in the motor when at idle (stoplights). It never dips to the point where it will stall but it notcieable enough - I can feel it through the seat. And a little electrical glitch to be found...when someone sits in the passenger's seat the airbag engages like it's supposed to, but when they get out of the car the airbag stays engaged.

The car is due for service at the end of March so I guess I'll soon find out. Any ideas how much the dealer would charge for bearings/ball joints (parts and labour)? I am out of warranty (except for the "lifetime" engine warranty they threw in for free but I don't plan on getting much use out of that )

Cheers

-2006 GTP Coupe/3900 V6/4 MM