Freelander 1 1.8 Resonance & other faults

So the Tangiers 1.8 is back on the road after much refurb work. Now using the engine for a while to see how it fairs.

All runs ok, but at about 4k revs, terrible resonance. Anyone had this? Is it perhaps a blocked cat or exhaust defect ??
Definitely the engine making the noise not suspension or transmission.

Check there's plenty of room around the exhaust, and check that all the exhaust heat shields are in place.

Not touching anything. Think it might be the new rear box sounding raucous.
I love debugging new (to me) vehicles. Two new ones on me.
1. Press brake pedal repeatedly, to adjust handbrake, and engine revs up to 2k rpm.
2. Whilst stationary, steering turned on full lock and against end stop and engine stalls.

Ideas ?!?!?
Watch this space.

I think I'd be checking the IACV is free. That could be responsible for both those issues.

Interestingly the IACV can be heard motoring itself more than I am used to!
Also, engine rpm at idle is hunting 650 to 800 ish.
All plugs are a good colour and no misfire detected. Vehicle seems a bit sluggish too.

So far, nothing to suggest head gasket has gone. Oil leaks sorted with a couple of new cam seals.

Will strip and clean IACV or replace it and see. Many thanks.