Noisy heater fan

I have my heating fan on full blast all the time, generally pointed at my feet to give engine most cooling it can. It runs just fine but the fan is sometimes more noisy than others. It will be blowing OK with expected noise at full blast then it suddenly changes as though its flow has changed then it clears and goes normal noise again. Any ideas what may be causing this effect. As its right behind the dash I'm not in a rush to take it off to investigate that far but just curious as to why the sound changes.

In all honesty I haven't stripped one of these fan motors down, but ones I have done have a plain metal Bush or a phosphour-bronze Bush, which is sort of, self lubricating, or even in extremes, a tiny ball bearing race.
My guess with the Hi-Jet it is plain metal Bushes, so you have a metal Rotor shaft running in a metal Bush. Skylark has been standing around for quite a bit now during a rather wet winter so both bearing surfaces will have rusted over the period of time. When you run the motor up the two rusty surfaces cause friction and will setup a slight oscillation or vibration and cause the screeching noise you are hearing. The friction causes the motor speed to decrease a bit.

Usually a bit of prolonged running will " smooth things out". I find that if you drop the fan speed down so the screeching stops and then speed it up again and repeat that each time then the two bearing surfaces, shaft and bearing will bed in again. On the other hand if it gets worse, you may have to resort to fan motor out. Not nice.

Could be the switch connection. Mine's only has full speed if i wiggle the lever till it makes contact.

its not screetching as such, more of a sound like you can get if a leaf has been sucked into it, then after a while it clears? Could do with one of those long cameras on a flexible probe to look at whats in there. if all else fails I'll fall back to the usual solution...turn radio up louder!