Just changed the auxiliary drive belt on my NC Sport today. whilst checking the pulleys, I noticed that the tensioner pulley has very slight side to side play. It’s not a lot, but it definitely has some. The car is 11 years old, but got a fairly low mileage of 40K. There are no abnormal noises present in that area of the engine. I just wondered does this warrant a replacement, as I’m guessing that a new one would have no play at all. They are not that cheap, the replacement part, though I might go for a decent pattern part, if they exist. Thanks for any help.
If there are no adverse affects then I’d leave it. I’ve never changed a tensioner for having play before, but I’ve not checked any for play either. I’ve only ever changed the tensioners for being noisey at idle, the noise was causes by the spring adjustment part of the tensioner not the pulley itself.
Thanks for taking the time to replay Robbie, mush appreciated. I think I’ll just monitor it, thanks again
A lady at work had a rattle at low revs, the sort of rev range when just setting off or reversing in to a parking spot.
Turned out to be the belt tensioner.
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Thread update 
I have had an intermittent rattle for about 3 months, it seemed to go when the clutch was disengaged pointing to maybe the release bearing. I did get it on low revs reversing, that is how I came across this thread, and the local clutch place diagnosed it as so. The clutch was going to be £650 fitted, not an inconsiderable sum of money. However I decided to try the tensioner and idler pulley as suggested by Keat63, replaced them this morning, now totally silent. Even the 2000-2500 rpm rattle has gone, that plagued me and I was sure it was gearbox problems.
All in all £48 plus my own labour, happy as a pig in sh*t now.
If you have a low rev rattle, or even 2000-2500 rpm one try this.
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Ironically my own car has now developed the same rattle, its on my list of winter jobs.