Slight whistle from boiler

[Dingdangdo]

Hi. I have a Baxi Platinum 40 Combo which is about two years old. Noticed in the last couple of days when the heating switches off the boiler still runs for a couple of minutes or so which is the norm I think. Once the boiler shuts down a short whistle sounds literally for a second or so. No other sounds on start up or during. Just when the heating is switched off and the boiler down cycle ends. I've got a plumber coming next week to change a rad where the system will be drained and refilled. Think the boiler has ten year guarantee but wanting to know what this short whistle sound is. I managed to record it so I'll show him next week. Will the heating still be ok to use until next week? Thanks in advance