I'm doing my best to get it!
The engine (any engine) can do this for two reasons:
1 there is something rotating or reciprocating which physically is or goes out of balance, and at some particular speed (the first harmonic) it really goes wild and starts lashing about. At TWICE that speed you might notice it again, but probably not so badly.
2. the engine is misfiring somehow, and making you feel the roughness.
Let's rule out 2 meantime, because you say it's actually running quite well.
Back to 1.
Start the engine, and sitting there parked in neutral, very slowly bring the engine speed up towards the bad range. It either will or won't set the vibrations going. If it does on what is very light throttle, you can be practically sure some rotating component is off-balance - like a two-part silly-arse flywheel for instance, or a crankshaft pulley-damper. If it was mine, I would hunt down rotating parts first. Some quite small part (like an alternator) could make a quite big vibration if it went right out of balance.
Next, if it vibrates when you run the revs up, take off the belt and try again. If it now doesn't vibrate, one of the things the belt drives is the likely culprit.
It's a bit of a guessing game.
If you cannot make it do this when the car is sitting parked, drive it again, and see if it happens at those revs in the gears, 2nd, 3rd, 4th, 5th, or if it only happens at a certain road speed. You see where we're heading? If it happens in most of the gears it's the engine. If it happens only at a certain road speed it's probably transmission or drive-train or hubs related.
Let's keep on the case because I want your Novacom thing.
I would pay for it ....
CharlesY
