TDI noise

Hi ive got  2000 1.9 tdi elegance, 120k  and i've always had a noise , say if i rev it gently then throttle off there is a noise just for a second or 2,  almost a vibration metalic noise, Ive put 12k on the car since ive had it and its always there but seems very slightly noisier lately.  Just had alternator tensioner and belt done and i think if there was a prob the garage would have said  .plus  the turbo works ok . been an expensive 2 yrs, steering rack, front spring rear spring battery fusebox window regulator, arb bushes and 2 new lower suspension arms ( last week with tensioner ) so I hope its nothing bad :(... bought it cos it was supposed to be reliable, any ideas ... Thanks