Rep Power: 0

Not sure if its a sensor issue or maybe the alternator is packing it in but when my car is at idle it slowly by steadily has a bit of a hum, or a whir, that deepens in pitch and then the rmp kicks up a bit and then slowly (over bout 40 seconds) starts to drop in pitch untill the rmp kicks up again.

The hum is similar to when the steering wheel is cranked all the way while idling and the power steering pump starts working to circulate but the sound is slightly different.

 
Rep Power: 208

are you getting any warning lights?

listen around the alternator and timing belt cover, it could be your water pump also, although that should not cause rpms to drop

 
Rep Power: 517

Unplug the 4 wire connector from the alternator so it can't charge. If the sound goes away then it's the alt causing it.

Does it stink too?

 
Rep Power: 0

I'll try the 4 wire connector tomorrow.

 
Rep Power: 0

Did the timing belt and replaced the water pump. The hump is gone and it's much more quiet but the rpms are still slowly bogging down and then kick up high.

Next up is to check the alternator.