Since the end of last summer my Subaru Impreza 2000 Turbo had the problem that it sounded like a diesel taxi when the engine was cold.

The sound was a real worry when it first started, it sounds rattly and very diesel like. Revving above 3,000 rpm made it go away and after the engine had run for five minutes it was just about gone.

Everyone that I spoke to, both online and at my Subaru dealer said the same thing. They all said it was caused by piston slap, which is apparently occurs when one or more of the pistons become slightly oval and bang on the cylinder walls until the piston expands when it warms up.

It appears this is a common issue on older Impreza engines but that it isn’t anything to worry about as long as it goes away when the engine is warm.

I did have one symptom that didn’t fit the piston slap case. Very occasionally, when using engine braking from 70mph+ down to zero, the diesel sound would reappear and go away when the engine was revved.

I had my 105,000 mile service the other week and asked them to take a proper look at the sound (I left the car parked round the corner from the dealer the night before, so that I could take it in with a cold engine).

Much to surprise I got a phone call later that day to say that it wasn’t piston slap after all, which brought on a brief moment of panic and thoughts of engine rebuilds. I needn’t have worried though, the problem was a faulty timing belt tensioner, which was banging against the engine block.

The tensioner had been fitted at my 90,000 mile service less than 12 months ago (when the timing belt was replaced), so it was replaced under parts warranty.

So my Scooby now sounds like it should again, no more taxi impressions every morning.