It only last around 2 mins, then it dies away.
Ideal boilers are rubbish. It could be the pump forcing the water through the divertor valve before its fully open. Nothing to worry about,just a bit of a nuisance.
Could be air in system, or low water pressure, have you checked it lately, should be a little gauge about 1 1/2 across, needle should be in green segment. Other thing is, is the condesate pipe clear.
If it goes away after its run for a while then don't bother.
