Rattling/knocking from Baxi Combi Instant 80 HE ?

My 93 year old mother-in-law's boiler gets a very loud knocking/rattling from the central heating system when the system is idle. The central heating side of the system is at present switched off.
The hot water is on.
The system has been power flushed.
It doesn't make any noise when the hot water is running.
We have had a heating engineer out on a number of occasions (the original installer) but he has not been able to 'fix' the problem.
Can anyone tell me what the cause is and how it can be alleviated please.
Thanks in hopeful anticipation