Alternator belt squeal, or not?

[redbull_J]

Dear Forum

I have a loud squeal coming from my 1978 non turbo 300D. It is most apparent immediately after start-up when the engine is still cold, and it only squeals when the engine is revved or put under load.

It used to disappear just a few seconds after start-up, but in the last couple of weeks the squeal continues for at least the first few minutes of a journey (but again---only when the engine is revved or put under load, never when it is idling). It still seems to disappear when the engine is really warm, though I am not taking it on long journeys to find out right now, til I have more idea what the problem is.

I had an earlier problem with my (York) AC unit causing belt slippage (I suspect the unit is on its way out, perhaps bearings worn), but solved that by adjusting pulley location and hence tightening the AC belt---no more squeal from that belt. I don't think that tightening that is the source of the new problem, as the new squeal is now there whether AC is on or off.

So I am wondering if it is the alternator belt. With the AC belt, I could see that every few seconds the AC wheel was locking, and causing the belt to squeak. This time there is never a problem at idling, so it is hard to check this one on my own, but I looked at the belts today with someone else revving the engine. The squeal was there for sure, but nothing was obviously stopping, or locking up.

Any ideas for a diagnosis? What should I look for to see if it is the alternator? The belt is I think fairly new, though tension is not super-high. Would low tension cause a squeal like that, even if there is no really obvious slippage (like with my AC stopping completely)? Would it make sense that the noise disappears when the engine is warm, and has gradually gotten worse over the last few hundred miles? Or is there some totally different diagnosis I can check for, nothing to do with drive belts?

I can give more precise details of the noise/symptoms if there are things I can test out.

Many thanks!