My RWD project car is making a knocking noise underneath. It's hard to say if it is the front or back. The noise is only quiet, only happens sometimes and only below about 15 MPH. I've replaced the propshaft and that made things a little better. I've inspected the rear diff mounts and suspension arms. They're fine. I can't feel anything through the gear lever or steering wheel. The noise stops when the brakes are on. What might it be and what should I try looking at next? Thanks.

Sounds like backlash in the diff to me. Is it BEC or CEC?

Strut suspension can give a low speed knocking noise if the struts are worn.

I forgot to say that the frequency of the knock rises with road speed but is less than 1 knock per wheel rev.

The car has dual wishbone all round, so no Mc Pherson struts. I guess it's worth looking over the springs and dampers.

Tooth missing off the diff ?

How do you determine that is less than once per revolution?
No t trying to be thick, but have you checked wheel nuts properly tightened?
Best regards.
AA

Check your wheel nuts are tight.