i'd be very surprised if the wastegate was jammed open, they have a heap of tension on them. i have tried blowing into a boost guage and i think my max was about 3 psi so blowing into the actuator probably won't tell you much

I think you should check the waste gate, the actual gate might have worked itself loose from the shaft that is connected to the actuator. That may explain the buzzing, because it's possible that it is now flapping around like hell when you give it stick. Also, the actuator might not be stuck open, but the spring might be screwed, so that it doesn't keep the gate closed if it even reads a tiny bit of boost, so it will open immediately.

And the winner is.........

Those who thought the wastegate was jammed open were the closest to the mark.

Luckily - It was just that the circlip that holds the linkage from the actuator to the wastegate on had come undone & the wastegate was flapping in the wind.
Cheapest turbo repair I've ever made.....: - )

I know what do next time I book it in for work & the guys in the shop get that "I'm going to take your pride & joy & thrash it" look.
Un-hookie the old linkage.

I've fixed it, but the buzzing's still there..... Only happens at 2000rpm!
Any thoughts what this could be?

did you use a new circlip?

Damn I had exactly that happen to me... and you will not guess where.... of all places, it happened to me while staging up against some turbo'd Charade at a local drag meet... man that sucked.... this POS 3cylinder crapped all over me....

worst of it all, I fixed it, in time for the next round, so my car was going good again.... this just made every one think this Charade really was that fast....