Engine/Drivetrain Vibration Issues

[jondee86]

If the rattling is heard in all gears, and even while the engine is running
in gear at constant 3000 rpm without deceleration load, then it is not
likely to be a torsional resonance It will be a simple vibration of loose
parts.

I had a rattle that started at 6000 rpm as if a switch had been turned on
Sounded like all the valve shims had fallen out !!! But in the end
it was found to be the clutch hydraulic line banging against the firewall.

As you say, when you start stiffening the car up, many things can cause
vibrations. Out of balance wheels, rigid or super-stiff engine mounts or
transmission mounts, spigot bearing missing from crankshaft etc. You will
have to use a process of elimination, checking every loose or flexible
metal part, that could possibly vibrate against another metal part

Cheers... jondee86

PS: I hope you you had both rear wheels off the ground when you were
running the car in gear looking for the rattle !!!