I was driving home (~100mi from where I spent the weekend) and ended up having an intermittent, low rpm vibration for about the first mile. it felt and sounded like someone had a 15" sub driven by a 1000 watt amp (low rumble, sorta like a cheap car with a HUGE stereo and a ton of bass), but it was rpm-dependent (vibration got louder/more intense as rpm went up) up until about 2000rpm, then it mostly went away, and appeared again around 3K, then eventually went away completely, was not speed-dependent at all (happened at just about any speed between 0mph and 50mph), and hasn't come back in the 100 miles I've driven since. I looked at the harmonic balancer and it has SOME minuscule cracks in the rubber, looked like a tire that had just barely started to dryrot, but there was no tearing of the rubber or separation of the rubber from either the hub or the outer ring, so I don't *think* it's the balancer... the car has about 108k miles on it, and the only mods are a big front swaybar, powerflex LCA bushings, MSD coil and plug wires, a K&N filter, and EBC brake pads. I'll probably replace the balancer as preventative maintenance in the next 3-4k miles, but other than that, any ideas what could have caused the vibration? I have noticed the clutch is slipping a bit under high load in the higher gears at highway speed, but that wouldn't cause a low rumble like this, would it?