Rattling noise when accelerating at low RMP

[Vinivine]

Hello guys,

Ive got a problem with my car making rattling noise when accelerating, but its only making that noise when at low RPM like 800-1k (Pretty much only when taking off or moving slowly). When engine is running at higher RPM the noise is gone. Also when car it at idle, there is no noise either.
The noise have appeared suddenly, short after I turned on AC. I haven used my AC since last summer. AC is working fine though, which I assume the clutch and compressor is fine? But the noise is regardless if AC is on or off which I was reading may not be the source of the problem.
I already checked the heat shield, engine mounts and auxiliary belt tensioner, all are fine. I had a throughout check if nothing is loose in the engine bay all around.

My car is Mazda 6 Diesel 2.0, year 2008. Mileage 149k

Any ideas what could it be?

Many thanks.

Edited by Vinivine on Wednesday 26th May 12:30

[Vinivine]

Hi guys just an update. I took auxiliary belt off and started car without it for a moment. The noise is gone. I am almost certain its the AC compressor. Unless its the alternator. All pulleys are fine.

Do you think I have to change the compressor? Or just the clutch?

[E-bmw]

If the air con is off the compressor isn't turning, so if it is always making the noise is is the clutch.