Overheating when heater set to cold...

Hello guys, hope you're enjoying this fantastic British weather, doesn't stop me driving my tf that's for sure ;) i have an mg tf 135 03 plate, I had a suspected hgf with symptoms being mysterious coolant loss with no visible external leaks and sludge in the expansion tank. Then I got the dreaded rising temp gauge and all the steam, popping and boiling :( Got russell down (absolute legend, I'd pay for his company he's so entertaining) and he diagnosed a dodgy coolant cap and a busted thermostat, hg fine! The honesty, expertise, humour and empathy from this man is a one in a million, i couldn't recommend him enough. 400 miles later since the fix and she's good, coolant hasn't dropped a mm and the car performs as normal HOWEVER... She over heats when my heater is set to cold, and when the car is at operating temperature and idling I hear a high pitched, loud noise coming from what I think is the engine area but it's hard to tell. Upon accelerating the noise seems to disappear. If the heater is on hot she doesn't overheat regardless of driving conditions, idling or taking her to 6750. Now Russel isn't the most available dude as he's extremely busy as I'm sure most of you know; hardly surprising due to his excellence. I guess my question is... Any of guys know what this could be? And can I still drive her about?? Thanks in advance guys, much love for reading.

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