Deep/loud grumbling noise when hot tap is opened

[stratplus]

As above, not restricted to one tap and intermittent. Unvented cylinder, just replaced multiblock and had new expansion vessel. Pressure 3 bar after multiblock, 4 bar before multiblock and expansion vessel checked empty at 3.25bar.
Only seems to be on hot outlets.
Any ideas?

 

[stratplus]

Noise seems to be coming from the cylinder cupboard.

 

[lame plumber]

something to do with the replacement multiblock would be my first point of call. Long screwdriver to an ear is needed to trace the source