Weird knocking/dripping noise when idle (2.0TDI) [VIDEO]

Sounds like alternator clutch pulley. They are designed to freewheel so the alternator can continue spinning when the revs drop (gear changes and the like).

When they fail they make that clicking noise. The tensioner that you can briefly see wobbling back and forth gives it away too. I changed mine and the belt whilst I was at it for the sake of a few quid. It's now silent and the tensioner sits near enough perfectly still at idle.

Edit:

It could be one of two clutch pulleys, (you can tell by counting the ribs on the belt)
Darkside Developments - Alternator Clutch Pulley for 1.9 & 2.0 TDi Models - 6 Groove £25 +VAT
Darkside Developments - Alternator Clutch Pulley for 1.9 & 2.0 TDi Models - 5 Groove £40 +VAT

Any motor factors will have the aux belt (match number of grooves again), I think I got mine from Eurocarparts for about £8, and it's a continental one too which was a nice surprise

It's a straight forward job if you know one end of a spanner from the other. You will need a special tool to get the pulley off the alternator though, like this one : Alternator Pulley Removal Tool Star Torx T50 x 1/2"D | eBay

Tension is taken off the belt with a 17mm spanner on a square block on the tensioner, and levering it off the belt. There are two holes in the tensioner that line up when you've got it to a certain point, you can put a drill bit through them to hold the tensioner out of the way whilst you fit the new belt.

I actually ended up changing the alternator because mine had seized to the point where when taking it off it sheared the alternator input shaft.

i think my car has the same noise only quieter and not as constant could someone please check for me. its louder and noticable more around the time im.closing the bonnet back but you can hear it on and off throughout the video.