I had a new combi boiler installed just over a year ago and I have noticed that the panel radiator in my living room has begun making a worrying sound which I can only describe as being as if iron filings are running through it, a sort of crinkling sound. It is intermittent and probably has something to do with the flow of water as it's being pumped around the system. I have cleared all air from the radiators so it's not that. Could it be rust residue trapped in the radiator? It's over 20 years old. Any thoughts on what it could be?