Probably mildly insulting to suggest or query if you have checked your
lug bolts for tightness?   So obvious, but sometimes we miss the obvious...

 I experienced similar sounds during a race practice once with my P-car.  We
scratched out heads until I figured out that no one had torqued down one of
the 12x18 rear wheels.  From that day on I began tying a piece of survey
tape around my steering wheel and only pulling it off after every lug nut
had been checked...Can you imagine losing a wheel at 150mph in traffic on a
race course?

  Also, you might look closely at your tires for bulges, loose pieces of
tread or un-seated places on the bead.