What's causing this engine noise?

[Graham.]

59 plate Octavia petrol. Only does it when warmed up.

TIA

[The Natural Philosopher]

sounds to me like some ancillary like a water pump or alternator has issues.

Or it might be an injector pump.

But the cyclic nature of the noise is puzzling. As if something were being governed and not directly coupled speed wise to the crankshaft, which remains at constant speed as the tacho shows.

Is this a turbo?

how do the cooling fans work..electric? fluid clutch?

thinking a bit more about it, the sound pitch changes quite fast, so if it is something being spun up and down, its happening pretty fast. so its possibly quiet small. smaller than a main cooling fan anyway.

MM. Aircon refrigerant pump perhaps? Those tend to go in and out according to some kind of switch?

[Graham.]

Thanks TNP, it was aircon related, I checked and the aircon was off turning it on the whine became continuous but barely audible above the engine. When off it still pulses at 2Hz but again barely audible so I suppose the clutch was in an intermediate state causing that loud racket?

Thanks again.

[Bob Minchin]

Have a look at the voltage on the aircon clutch control and see if that is pulsing too. It maybe a fault in the aircon controller. The clutch drive signal should be a simple on/off voltage of zero volts or battery voltage.

[Graham.]

Thanks for that, it's a company car and its replacement is already ordered. My reason for posting was to obtain reassurance that the engine wasn't going to seize up in lane 3 of the M6 in the interim.

[The Natural Philosopher]

I feel a bit smug.

it still should not do that. The clutch should be in or out, and with aircon off, out

I think you need a new clutch there. Or a new driver for it. Maybe some damp has got into the control system.

Mm. I'll stick my neck out here. the clutch is in fact dragging. then slipping..so its the clutch almost certainly...

Oh bosoms. It looks like that's a whole new pump, best part of 300 notes,. fitted..

warranty?

[The Natural Philosopher]

good plan

+1

[The Natural Philosopher]

Ah. SEP. Someone Elses Problem.

Nope. The only thing to check is where same belt that drives the aircon also drives something else, cos if that pump seizes the belt will go.