Hotpoint Aquarius TVFS 83 double beep when drying

The tumble dryer is about three years old. Gets fairly heavy use and I have had to replace the ear bearing recently.
Its a standard non condensing tumble dryer
When drying it sometimes gets in a mode where there is a double beep every 2 or 3 minutes but it continues to dry.
At first I thought it was because the vent filters had some fluff in them as switching off and cleaning cleared the issue but I have now had instances where the vent filters are clear and the power cycle still cleared the beeping.

Would be great to know what the beeps are indicating, I assume some sort of fault?

Thanks

Alan