ZX2 Reversing Noise and coolant

i have a 2003 ZX2. Recently it started to make a clicking noise when i back up. i lifted it but there was nothing that seemed out of place or loose. It sounds like spokes hitting something. It drives fine and the noise only sounds sometimes. Any ideas? Also how much coolant is it supposed to use. I've had other cars and the coolant ususally lasted a month or more before it dropped inlevel, but The ZX2 seems to need it every few days. There doesn't seem to be a leak, I don't see any on the floor when i park. any direction you can give will be greatly appreciated it.

Regarding the coolant level:
Take a look at your engine oil via the dipstick. If there's a chocolate milkshake colour to it you likely have a headgasket issue/leak, and that's where the coolant would be migrating to. Not a good thing at all.
I had an '88 Oldsmobile with a QUAD4 engine which was notorious for this problem.

Hope this isn't your problem, but then again I hope I helped steer you in the right direction if it is! ;)

Also check your coolant reservoir tank, is the coolant contaminated with oil?