After driving for about 20 minutes, when the engine (oil) temperature is 1/2 of the scale I have a strange noise. On OD (but I get the same thing in other gears also, but less noticable) beetween 2000 and 3500 rpm (maybe even higher, don't know) I hear a tickling / scrathing noise. When revs are higher this knocking gets faster, so it depends on rpm speed. But when I left my foot away from acceleration - it disappers completely. Could it be the exhaust leak? But why I don't get it on cold engine/exhaust pipes and only when I accelerate (or keeping the constant speed)?

 

Most likely an exhaust leak...

 

A tickling noise? Or maybe a ticking noise? I have never really heard any noise from a tickle.