Technical knocking sound under the bonnet

[Trancendental]

Hey all, more questions for you

I have a knocking sound which sounds like its coming directly from the engine but its not coming from the same place with each knock, its like its moving from left to right but its constant. Can hear it from inside the vehicle when moving, cannot hear it when stationary.

Only happens at certain engine temps and at certain speeds, if a mechanic asked me to show them I'd have to recreate the conditions some how.

Anybody got any ideas?

Thanks