2005 Mazda 6 sounds like a flat but is not

i just had 4 new tires and rims put on - aug 2009 - started driving the day after thanksgiving and it sounded like i had a flat in the front - i pulled over -no flat - air pressure was down on all tires (24 should be 32) so I fixed that - but now still at accelerated speeds (over 70 mph) i get the flap - flap - flap sound that I have a flat tire - any suggestions? it goes away as I go slower