For the past couple of months I've noticed a whirring noise from my 1993 3.2L petrol Bighorn. My garage has been unable to identify it. Here are the details:
1. The pitch of whirring varies with engine revs - this is so obvious when pulsing gas pedal that anybody could hear it
2. The whirring disappears once engine reaches running temperature
3. It's not the AC idler pulley as garage changed that as it had a noisy bearing
4. It's not the alternator as garage changed that because brushes box got burned up by oil getting in somehow and burning (2nd time in 4 years but that's a different story)
5. It's unlikely to be a belt noise as it doesn't change when I spray water on the belts (a friend suggested I try this out)
6. If it's an exhaust manifold crack, then it must only be sucking air in as a CO monitor shows no CO accumulation under the closed hood
7. This may be unrelated but, after starting, the exhaust sound varies from sputtery to normal every 5 seconds or so - could this be related to (6) above with air being sucked in and then fooling the O2 sensor?
8. This may also be unrelated but, when cold, the engine sometimes misses on accelerating for the first 500 metres or so (misleading O2 reading being sent to engine computer because of 7 above?)
9. Can it be a timing belt, or timing belt tensioner, issue? - both were replaced 6 months ago - and would this quieten down when the engine warms up?
All suggestions will be very much appreciated.