Mine does the same, but it isnt your wheel balance. This is a tech note;
#PIC4989: Drive Line Boom Noise And/Or Vibration - (Oct 2, 2008)
Subject: Drive Line Boom Noise and/or Vibration
Models: 2008-2009 Pontiac G8
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The following diagnosis might be helpful if the vehicle exhibits the symptom(s) described in this PI.
Condition/Concern:
There may be a either a boom noise or vibration and in some cases both a noise & vibration detected in a very few 2008 & 2009 G8's. This condition may be detected by customers at speeds of 55-65 MPH on either 08 or 09 G8's. The noise could be described as a low frequency boom or moan noise (not a whine) coming through the body. The vibration is generally felt in the seat or the floor pan. The condition could be caused by either a misaligned center support bearing assembly or the prop shaft mounting to the differential yoke.
Recommendation/Instructions:
The prop shaft center support mount is slotted left to right and can be adjusted by loosening the bolts which are accessible through holes in the metal shield between the prop shaft & the floor sheet metal. After loosening the center support bolts you can use a pry bar between the prop shaft & floor to adjust the center support. If the alignment isn't correct the prop shaft will usually "snap over" into proper location with very little input.
If the condition is both noise & vibration felt through the floor, re-indexing the prop shaft to the differential yoke may have to be performed:
Mark the relative alignment of the propshaft, rubber coupling, and differential in the original position.
Number the nut and bolt pairs and their hole in the rubber coupling which join the rubber coupling to the differential (silver) and the nut and bolts which join the propshaft to the rubber coupling (black).
Remove rubber coupling from differential and propshaft, ensure the nut and bolt pairs are kept together.
Re-assemble indexing the rubber coupling and silver bolts clockwise (relative to differential), and the propshaft and black bolts counter clockwise (relative to differential).
Please follow this diagnostic or repair process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed.
