My 2002 S3 has a very slight clatter when I first start it up from cold. It sounds a tiny bit like diesel for the first 1/2 second. Then it quietens downs and runs very well. Could this be because the car was used to not being driven much before I bought it? (last few years 3k) When I had it serviced they used an engine flush and I have now go it on fixed serivce intervals using mobile one. This has made it much better, but is this normal?