I need some help. I have a 95 2-door Integra GS-R and I'm getting a lot of noise from my tires. It sounds like a "warble", or " wa-wa-wa-wa-wa-wa" and increases as my speed increases. The strange thing is that once I rotated my tires, the noise got A LOT worse. I rotated them as required, front to back because they are directional tires. There is a little vibration around 15-20mph (independent of engine speed), and the steering is fine, straight with no shimmying.
The tires are Dunlop SP Sport W-10, P 205/55 R15 87V. These are on the stock aluminum rims.
Any ideas on what this may be? I tried messing with tire pressure to no avail. Could this be due to improper tire balancing?

Please help!