Tire noise.. help!!

I'm getting a loud hum and vibration from my 96RS. I figured it was all coming from the rear of the car but I've noticed the vibration up by my feet while driving and while riding in the passenger seat. The tires I have now are Futura (not the right size either.. I think they are too tall). Not sure how much this brand sucks. The car sits during the winter and the tires have seen two years so far. I noticed tonight the pressures were low. So I put 35 psi into them and noticed that the hum wasn't as loud. No huge differance but some. Could these tires have gotten a flat spot in them over the winters? Could this be the cause of the hum?

Thanks

JB

Are they cupped any? Try rotating them to see if it helps or feels like the vibration is comming from another location.