Long story:
No annoying vibrations from the old stock Michelin tires on my '03 RSX-s. Got 4 new BFGoodrich g-force super sport tires mounted on my stock undamaged aluminum rims. Noticed a cabin vibration (barely perceptable on the steering wheel) beginning at about 48mph and diminishing at 55mph. Then again at about 70mph. Went back to the installer. All 4 wheels were off about 1/4 ounce. Rebalanced (on spin balancer) and vibration was less, but still present.

Second visit the owner drove the car, said he didn't feel anything that bad (he drives a truck), rechecked the tires and said they all balanced perfectly.

Went to another place for rebalancing with a road force balance machine. One tire was at 6lbs, the others about 4lbs. Road tested the car and the vibration was almost non-existant until I hit 70mph. At that speed the shaking could be felt and the steering wheel had a slight side to side twitch.

Front and rear end components show no signs of play or looseness when normally loaded or with the wheels in the air. The steering rack seems tight with no slack in it.
The struts rebound equally and don't bounce.

Tire dealer probably won't warranty tires that don't show a defect, but I still have an annoying vibration problem. Short of throwing more money at this problem, what would you recommend? (I'd like to scrap these tires and try another brand at this point.)

One final note, at a slow speed (20mph) across a smooth surface, I hear a 'whoosh-whoosh' sound from the tires. The tread is noisier than the Michelins, but what concerns me is it isn't steady.

Hey, thanks for suffering through the details and providing any input you may have.

 

Have the place you balanced it check if you have a shifted belt on one of the tires.Also try rotating the tires to see if the problem keeps occuring.