Before you chase down the 'complicated' stuff, also look into the more simple(and not so durable) items like the vacuum pump.
Notorious for clicky/tappety noise and puffy/sucking/choofing/noises.
Noises can deceptively 'travel' or bounce around and while you think it's coming from one point, it turns out it's coming from somewhere else.
I had this issue on my D1 tdi, and drove me spare! ... loud clicky/clacky noise, and I was sure maybe a tappet or lash adjuster. Seemed to be louder on the passenger side of the engine, and more accurately about cyl 2 and 3.
Cover off and all valves had good clearance and all caps still there. Maybe exhaust manifold .. ???
Only place I couldn't point a stethoscope was near the front fan serpentine belt area, so off with the belt and motor idling ... and noise gone!
Maybe water pump had some crud inside it .. could start to understand what this loud clack could be. Belt back on but fan off, allowed me to turn pump and get stethoscope near by gingerly avoiding spinning pulleys, and noise still seemed to be pass side near cyls 2/3.
I thought to place slight pressure on serpentine belt adjuster and noise gone... and now I was getting closer. tighter belt ... noise gone. adjuster maybe shonky. new adjuster ... same stupid clack noise! ... arrgghhh!
Issue was so simple ... belt stretched just that bit too much the adjuster was bouncing on it's end stop.
Fitted a new belt, and with both old and new adjuster fitted, no clacking
The point isn't that you don't have a valve or cam or whatever issue ... just that sometimes the issue is so simple ... and easy to eliminate from the probability list.
That is: you have a noise whilst engine is spinning over. You can safely run the engine at idle for a few seconds with no fan or fan belt .. no problems.
If you do this, does the noise change? If so then issue is going to be more simple than you may have originally considered.
Another thing to keep in mind .. you could have multiple issues. The sucking like sound could also be gasket on inlet side leaky, or intake hose loose or something, and the tappety noise is something else.
Arthur.
'99 D1 300Tdi Auto
'03 D2 Td5 Auto
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