weird tapping noises

[NotABrick]

Hi,

I have a 2004 S60R, 6 speed manual with about 125,000 km on it. A couple weeks ago, I started to notice a weird tapping noise coming from the front of the car while on the highway. It sounds almost like a hollow plastic noise. It's not always there either, it comes and goes at random. I brought it to a mechanic, but they could not duplicate the noise. They've taken a look under the car, brakes, suspension, etc. They don't see any obvious damage or wear on anything.

I've noticed that the tapping tends to get faster when I speed up, but only to a certain frequency, then stays the same no matter how much faster I go and will eventually go away as I drive. The noise also seems to get worse when I lift off the gas, but gets better if I press my clutch down. Turning doesn't seem to affect the noise at all.

That's all I can think of off the top of my head. Can anybody help me out here?

Thanks in advance.