So I change the main jets and get ready to take her for a spin. I blip the throttle and ........ holy crap - what is that vibration?
The whole tinware seems like it wants to leave - somethings not right.

I think the VW Gods are trying to send me back to the office. Perhaps I violated one of the laws of VW mughanics?

What could this violent vibration be caused by?

It has a brand new Bosch alternator and serpentine belt on it.

Terry - Offender of VW Gods.

seeing that you have had a new alternator fitted, i suspect the fan has not been tightened and is coming loose the easy way to find out is to take the fan belt of and start her up. it could also be that the belt has been to tight and has buggered the bearing

maybe its the extra ponies trying to get out ?!

Yep - took the belt off. No vibration.
Spun the alternator - bearings seem smooth, no endplay all good there.

So here is the answer. I am a moron.

There is a polished stainless panel on the back of my engine bay. It has a sharp edge. I put a rag there to stop my hand being cut. I left it there.
The fan sucked it in.

All good - everything works again.

ouch Rags and air intakes dont mix do they Greedy

get a mirror and use it to see if u have got all the bits out of the fan

these will go through and sit on top of cyclinders and cause hot spots

not very good imn an air coled motor

By some minor miracle the rag is complete!. There was no shredding or jamming or other such complications - to my knowledge.

Thanks for the great tip. I would not have thought of that of my own accord.

yep pod now i will have nightmares, and crawl back under the bed ,all that money well $3.00 wasted