Serp Belt area noise and tensioner variations

[nrobinson]

I’ve just begun investigating chirping and rumbling noise from the v belt area on my 2003 XC90 d5. Interestingly its much worse at idle – a few revs and it quietens down.

I’ve removed the tension on the belt but not removed it yet, but already suspect I have at least a failing power steering pump as there is play in the pulley / shaft.

I’ve not checked anything else yet including the tensioner, but whilst gathering info on what needs to be done stumbled across a number of worrying discussions, including some on this forum that discussed a problem where the serp belt can jump the bottom crank pulley, go through the cover and basically trash the engine when fouling the timing belt. The discussions were quite old but did suggest it was a common problem, so my first question is, in the passage of time is it that common!?

My next question which is related, is about the tensioner. From what I can see from the discussions I’ve read is that if the belt jumps its normally as a result of a failed tensioner when the spring has weakened and the pulley forced at an angle. One of the threads seem to suggest there is an older type of tensioner that was more prone to this. Is this correct?
At the moment mine looks ok visually

From what I can see looking at VIDA there appear to be two OEM part nos for the tensioner:-
31251252 for engines up to number 349808
31251251 for engines 349809 onwards.
..which even that looks odd as my thinking is the pns would be the other way around based on age.
Is anyone able to tell me what the difference is on these two options?

Thanks
Nick