Noisy radiators

[colmac]

From time to time I get a loud throbbing noise coming from a group of 3 radiators. If I turn the thermostatic valve up slightly on one of them, the noise disappears - but usually returns a bit later. Occasionally, turning one of the valves down slightly can also stop it (this only applies to one particular radiator).
My central heating pump has 3 speeds. It's currently set on speed 1 (lowest). If you turn it up to speed 2 then you get the throbbing almost immediately.
Any help/advice gratefully appreciated.
Thanks.

 

[bab]

this noise is usally discibed as banging or knocking when a trv is shutting off or turning on . if its a 1 way stat it might be on the wrong way or if its a 2 way might just be faulty. look on body of valve & see if it has an arrow or even two arrows . then turn off the trv let the rad cool down with the heatig on. open the trv & see which pipe gets hot 1st. then post back

 

[colmac]

Thanks for this. Of the 3 rads it looks as though two have 2-way trvs (a right-angled arrow) and one has a one-way trv (single arrow pointing towards the rad).
I turned off all the trvs and allowed rads to cool. It looks as though the 2-way trvs are on the correct ends (i.e. when trv opened, water temp in pipe next to trv is much higher than in pipe on other end of rad). However, I think the one-way trv may be on the wrong end. When I open this up, the pipe at the other end of the rad is hotter than the pipe next to the trv.
Could this be the cause of the problem?

 

[bab]

ya right about the two way stats. the end of the rad that gets hot 1st is the flow. it dont matter if the trv is on the flow or return as long as the flow of water is in the same direction as the arrow on a 1 way stat. u may be able to turn it around if it has 15mm compression joints on both sides or leave it turned up to max 4 the time being.

 

[colmac]

Thanks again. This has been really helpful. I'll see if it can be turned round - or possibly replace it with a 2-way.