Ideal Classic RS40 Shhhhhh... cycle shut off noise

[waternightmare]

I have just moved into a house with an 12 year old Ideal Classic RS40, it appears to be working fine but when the boiler cycles on its stat it makes aloud shhhhhh noise before it clicks off and returns to pilot.
The length of the shhhhh noise varies from time to time but can be a for a few seconds, it sounds like something unwinding and I have discovered that if the timer or room stat shut down the boiler, it "clicks" off instantly without the noise, also appears to fire up on demand okay.

Any ideas? Pretty simple boiler from what I understand, no fan, pump, PCB or clever stuff in there so I am thinking along the lines of a valve struggling to shut or is this noise completely normal just worried somethings on its last legs.

Boiler stat set at 3, system is microbore, open vented, pumped with 10 rads/copper cylinder via mid position.

 

[Agile]

I would first suggest the boiler stat is failing to click off as intended and instead its arcing on the contacts. Probably giving a lot of radio interference as well.

Putting a medium wave ( MK ) receiver near the boiler would help to confirm if this is indeed the problem.

Simple replacement but as its a sealed boiler the heat exchanger probe should be fitted by a registered gas engineer.

Tony

 

[waternightmare]

Thanks Tony, will give the radio a go, thermostat knob feels quite stiff / rough to turn so don't know if that is connected.