I've noticed a strage metal sound (like tapping the exhaust with a scredriver) coming from either underneath the car, or towards the front.

I've heard it before, and it turned out to be something on the exhaust bashing the underside of the body at certain revs (I guess the vibrations were different).
This particular tap sound can be hear outside of the car, and it is extremely quiet inside the car (cant hear it when talking to someone), and also cant really be heard unless driving past a wall so the sound can rebound off of it.

I thought it was just the exhaust hitting again, but I noticed the sound happening at different times.
It happens after changing gear, when the clutch pedal is lifted theres 2 metallic taps (described above), but this doesnt happen if the pedal is lifted slowly, or very rapidly. I can reproduce the sound by letting off the gas, then putting it back on (same 2 tapping sounds).
It also happens when letting off the gas, as the revs drop below ~1500rpm, but this time its more frequent and random (when changing gear it makes the sound twice, and thats it).
I'm pretty sure it doesnt make this sound above 1500rpm, or I have yet to hear it, but below 1500, it makes the sound in any gear.
I can hear the sound even if the car isnt in gear.

When I got home tonight, I put the car in gear, and lifted the clutch slightly to see if I could hear anything else (which I didnt), but I did notice the sound happens when sitting idle (engine running).

I cant produce the sound if the engine is off, although I haven't really tried (maybe I should)

Anyone got any ideas on what it could be ?

The car hasnt changed in driveability at all, or I've not noticed any change. Gear change is still possible, and theres no struggle. The car accelerates with no problems and slows down with no problems.