PD with a sudden knock?

Got a friend with the 1.4 3pot tdi in a 2002 polo, we did the timing belt, full kit and proper tools and it ran sweet. A couple of days later he let it sit idling, after about 30 seconds or so it started running really rough so he shut it down. I've been back to have a look and it has a definite knock to it now.

The timing is still spot on and there doesn't appear to be anything obvious but it sounds like a valve is making contact with a piston unless maybe a lifter has collapsed.

Took off the valve cover to have a look and couldn't find any cracks on the tops of the lifters or that and they're all going up and down with the cam like they should.

Talk about a strange one.

I tell you what too, I've never seen an old belt so ready to break before either, it had cracks all the way round going across it.

Anyway I'm going back down at the weekend to start pulling it apart, it still starts up great and runs pretty sweet at idle, obviously not revving it as I'm not quite that stupid lol.

Going to take off the rockers and maybe the cam and lifters so far.

If anyone has come across this before though and has any suggestions on what to check then all are welcome.

Cheers

Yeh that's the other thing I wondered about, I know it's a problem on some but usually the oil light comes on when it goes. There's a few variations of the 3 pot and so far it seems to be the one in the skoda that fails as far as I know.

Just an update, the noise has gone! Looks like it was a sticking tappet and I think I know why, there was oil in the boot that wasn't the correct spec so it's getting a proper flush and some decent stuff as well as a filter.

Pretty pleased with that though, it was a hell of a racket and I really thought a valve had dropped.