Thanks to everyone who replied, lots of useful leads. Looks as if I'll be diving headfirst into the engine bay tomorrow. I'll keep u posted.

Hi

Tearing my hair out on this one.. Throughout the Rev range the engine will, the only way I can describe it, resonate, I.e sounding as if the exhaust is blowing, it does this intermittently even when stationary and in neutral. I thought possibly the alternator or turbo, but cannot pin it down ...