Every time I turn the cold tap on in out upstairs bathroom, the water come out then CLONK and it stops. Turn the tap on again and it CLONKS again. After a couple of times it works normally.
What is happening and how can I sort it?
I’ve tried turn the isolater in the pipe to reudce the flow, but it still does it.

airlock? try opening all the taps in the house and running them for a few minutes.

Dead body in the water tank

Baby Robin with a hammer in the loft ?

Sounds like an airlock somewhere. Not sure of the real solution but I would start with all taps running at same time for at least five minutes then turn off from nearest to stopcock. Good luck.

Not sure that I want to leave all the taps running for 5 mins, we’re on a meter….

Ees pigion in water tank ? Oww he get there ? Oink oink !

Apologies to Manuel.

Go with Matthews’ idea. Possibly ending with leaving the offending tap on full to ensure it is flushed.

Check for a high point in the pipe run if possible too.

Move to Scotland, no meters here !! Plenty of water too ! And Midges !

I left Scotland 15 years ago!
Not going back to live, too cold, wet, dark, midge infested…

incorrect pipe sizing can cause clonking in some circumstance I think.