weird noise while in reverse

[chevaps]

The other day i noticed a weird noise coming from under my bonnet while reversing. I can get the car into reverse fine, but as soon as i start to reverse, I get a grinding/humming sound from what appears to be the passenger side of the engine bay. Now the strange thing is that it happens most of the time, but not all of the time. Like yesterday while experimenting a bit in a car park, the first few attempts got the sound, but for the rest of the afternoon and night, i got nothing, no matter how hard i gave the gas.

Experimenting a bit, slowly releasing the clutch (with no gas) and then when fully engaged, i give gas and will not get the sound. However, when giving a bit of gas while releasing the clutch, i will usually get the sound. However, it will last all but a second or two and the car will reverse with only the usual whining sound of a manual gearbox in reverse. Before people jump to the conclusion that its the clutch, driving and taking off in the forward gears is perfectly normal with no grinding/humming.

I dont have the time to get to a workshop until the weekend, so im throwing it out there to see if anyone else has had a similar issue and may know whats up. By the way, the car is 2004 1.8 manual focus (with the ib5)

Regards

Tom