A4 quattro 1.9 tdi avante - metalic chatter at certain rpm clear by 2500rpm and above.

Hi. Happily bought the above vehicle with 150000 on the clock after owning an a3 for 4 years with the same 130bhp engine and liking it. Unlike my a3, the quattro has some funny noises! if i gently accelerate, at around 1800, i get a slight retardation of acceleration with a metallic judder. I can emphasise this by driving at that rev range slowly uphill. If I floor it, none of the above and take off is smooth and very fast. I'm already having the steering pump replaced, as that evidently IS failing, but I don't see the link with that and power reduction, briefly, at certain rpm. I've read a lot of threads hear and been helped to knowledge as a non member with my A3 just by reading the pages... really hoping this problem is familiar to one of those in the know.... Many thanks

Thanks Pauldazzle and Darran for both of these. ( Darran - sorry for ignorance,but what is the VNT? cheers)I spoke to my mechanic and took him for a test drive in it - he has suggested problems with the turbo too - hoses - valves - actuators as well as the turbo itself... he has the diagnostics to test audi..(not run that yet) I have no faults on the dash though. Pauldazzle - would that loom fault show up on the diagnostics? I did have that very problem on my a3 130 bhp and the same mech diagnosed that instantly.. This is a more detailed description of the problem:
2002 audi quattro 1.9 tdi. Rattling/grinding noise when slowly accelerating. Hits around 1800 revs can happen up to 25000 - 2600 then runs perfectly quickly. Easier to get the effect on a hill- under load with slow gentle acceleration. If I floor it there's no sound like that at all - not as fast as it should be in that rev range (below 3000) but no strange noise. Brief loss of power around 1800 commonly, but other revs too below 3000. The sound can "run on" after the revs drop to go to the next gear, rather than being tied to the actual rev speed. Sounds like 2 metal dinner plates vibrating together with a slight grinding noise with it. I also get misfire at low revs but with no sound - just a brief lack of power - enough to notice but not drastic. If you had limited funds.... where would you begin? .....Please don't say the scrappy!! I always wanted the a4 quattro, so I want to get it all into shape eventually and keep it fully maintained for the rest of my natural life...!