Please help..

Over the past couple of months i've been getting a strange buzzing sound coming from the front of the car.
The sound occurs in all gears including neutral. The strange sound can be heard when i press the accelerator pedal hard in any gear between 1800-3000rpm (the pitch of the sound gets higher with rpm). when in neutral and i gradually increase the revs there is no noise but if i blip the accelerator pedal i get the same strange noise.
I've had a look underneath the car and there doesn't seem to be any loose heat shield.

Any suggestions?