Avensis knocking

[kuksmith]

I have a knocking sound that is barely audible in 3rd gear, noticeable in 4th gear and very clear in 5th gear - in 5th gear at 1200-1400 rpm and again at 2200 rpm. I first thought it was gearbox, but since I can now hear it in other gears, not so sure. Piston Slap perhaps. Its not noticeable when ticking over or when not driving or engine under pressure - i.e. if running and I hear it, disengage the gear and no knocking sound. Clutch replaced a couple of thousand miles ago, but the noise was noticeable before than anyway.

Engine has 185000 miles, so well used but otherwise sound and always garage maintained. 2005 Avensis estate 1.8 petrol, manual. So far and good car.

 

Any ideas as to the cause?