Knocking in Radiators !

I have 3 radiators in my downstairs lounge. I can be sitting in my lounge with my heating having been on all day. Then all of a sudden one of the 3 radiators will randomly start to make a 'knocking' sound.

To remedy this situation i have to increase that radiators thermostaic valve slightly and the knocking stops instantly....Please help as ive no idea why this is happening or why it is randomly in any of the 3 radiators.

Im looking for a permanent solution please.

Do you have trvs on all your rads?. Have a look. You should have one rad left permanently open on locksheilds.
If not the pins could be sticking in your trvs. Take off head and gently tap the pins to free them off

COuld be that the trv is on the wrong end of the rad (Are they on the flow or return) or that the lsv is open too much. Try closing it a bit, or a lot.