Just had an amusing experience. Had a (non-baxi) guy come out to look at it and told him what you said. He didn't think it could be that and started talking about a dodgy flow switch. Having taken it apart and saying it was probably that was in the process of ordering one over the phone. Not...
I also have a baxi 105e with what sounds like a similar problem. I just want to check its the same issue though...
When the CH comes on (and possibly off) there is a whining noise coming from the boiler - which stops when you turn on the hot water. This is the bit that sounds like the issue...
