if my tires are underinflated by 5 to 7 lbs, wil that cause them to roar at speeds over 35?

This would add more surface area contact with the road, hence adding more friction. So yes, in theory there could be potential for more noise from the tires. I've never let my tires get below 30 psi, so I wouldn't know from my own experience. To put it into perspective, if you've ever rode on a flat tire, it can be very noisy. But if all 4 tires are running 20 percent or more below the recommended inflation, I would presume a substantial amount of noise would be audible. In theory, the anomalous sound level of all 4 tires running that low could be comparable to a single tire being 80 percent below normal pressure.