Drained the sump of my almost 10,000 k old oil this week. Quite surprised when after I had put in 5L at the beginning of the year, 4L only came out this time, after including oil filter and all. I didn't think it burn's oil, and theres nothing obvious like that, no leaks or anything.
So I've put 5.5L in this time, the engine wasn't noisy before hand, and now it's really smooth and fairly quiet except for a bit of crank noise at the bottom end. But what I'm hearing now it this loose pully vibrating kind of sound coming from the middle of the engine. Like a pully that has a bad bearing, and it makes a kind of noise much like the lazy front suspension noise you sometimes hear on these cars. It comes on for a couple of seconds, then goes away. Intermittent like.
I've never heard this noise before, and didn't think an engine that appears to be running this well, could make a noise like this.
Any ideas from the vague descriptions I've written here

its got nothing to do with oil change but my a/c pump started making a noise like that it comes and goes all the time

Yeah they can do exactly that. The air-con compressor might be on the way to seizing. It makes the squealing grinding bearing noise when they go. You'll be up for between 700 and 1000 dollars to have the system fixed and compressor replaced.
With my problem, I've checked over everything and I am stumped. I have a feeling though, that it's coming from my timing chain area under the cover, that or around the top under the rocker cover.
But the cars running alright, odd miss here and there but I did reset the computer recently.

Heres a funny thing though. I will start the car from cold, and leave it sitting there and stand outside the car. And all of a sudden after about 2 or 3 minutes of steady idle, the idle suddenly climbs up exactly as if I'd moved the steering wheel back and forth, but didn't. Then it takes 20 seconds for it to go back to steady again??????

Just a thought, maybe it is your lifters?

yeh thats a good point the lifters are bloody noisy in mine as well when i got the car i changed the oil and and it was making some scary noises after that . the old oil would have been thicker as well so it would have filled up the gaps in simple terms .

thats sounds liek the isc needs a good clean!

what oil did you put back into the car?
the lifters will be making a ticking sound

It had Penrite Premium 15 to 60 in it before. And I replaced with same. Although now I noticed they are putting Semi-Synthetic on the front label. The last batch of exactly the same didn't have that??

Idler pulley?

yeah check your pulleys.

I recently had my power steering idler give up the ghost.

Was F**k loud. It ended up going quiet as it seized and cooked my belt. I've just had my gf's dad (ex mechanic) change both pulleys and all 3 belts. Score!