Abnormal engine vibration above 4000RPM

[Rom]

Hi all,

I was just on my way to work this morning in my 1999 2.5i manual transmission US-spec Boxster and was cruising at about 3000RPM on the highway. As I approached my exit I dropped a gear and engine RPM's went to about 4500RPM (blipping the throttle, synchronizing the revs, etc). At that point I noticed a very abnormal vibration from the engine. I lowered the revs immediately and just cruised in to work at lower RPMs. When I got to work I accelerated up at rest (out of gear) and the vibration seems to start around 4000RPM. If I rev up to 4500RPM and then let go of the accelerator, the revs drop to idle, but at around 1200RPM there is a slight hesitation on its way down. That could be normal from the sudden vacuum, and different cars behave differently, but I'm pretty sure mine didn't do that before.

The oil levels are normal, and the engine only has about 74,000 miles on it. No warning indicators light up. I haven't got my OBD-II diag tool with me, so I'll run a code check later and see if anything comes up.

Before I jump in and start running diagnosis, does anyone know if this is a known issue with the 1999 2.5's, or have any idea what it could be? I don't think it's driveline related - the vibration occurs out of gear, at rest and with the clutch depressed (so the clutch plate is not touching the flywheel).

I'm guessing it could be a faulty spark plug (missing at higher RPM's), but then I'd have expected a check engine light and probably a slightly different feel. It could also be a broken engine mount, but again it doesn't sound/feel like that kind of vibration. It almost feels like an unbalanced crank or a big-end bearing problem.

Any comments appreciated!

Thanks!

S.

[Rom]

The vibration seems to have mostly gone away now, and OBD-II registered no warnings or faults.

I think maybe it's time to do a service... ;)

Thanks!

S.