I also have a 35L that does the same. One tip I can give is make sure after the boil, it completely stops boiling for 30 seconds.
I'm not sure if this is happening (haven't been bothered enough to check), but I work on centrifugal pumps at work. They don't like any air what so ever inside the pump, causes cavitation, and will never fix itself until purging the pump of air. Turning on and off in essence will help clear a little air pocket, if that's what it is.
For me, it only happens after the boil, not during mash.
Yes it is possible that if you have a very vigerous boil that bubbles from the boiler go back down the pump inlet hole and start to generate a cavitation effect due to steam bubbles.
As Morgz has suggested if you turn off the elements and wait 30 seconds this cause of cavitation might be prevented.