Have someone bump the idle until it makes the noise. Spray some windex, WD-40, whatever's handy onto the belts (don't shoot it into the alternator). If the noise goes away, replace the belts.
My car was making a chirping noise on cold startups despite having new belts put on when I installed the new engine. Two days before I bought tensioners for it, it threw both belts and left me on the side of the road. Luckily, I wasn't far from home and was able to drive back without a water pump spinning. I replaced both tensioners, idlers and belts again... and I still notice a chirping noise, normally when the car is warm. I can hear the chirp bounce off cars/walls next to me on decel at ~1500-2000RPM, as well. I'm beginning to think the alternator bearings are failing... but it's still charging and I don't hear the noise 90% of the time, so I'm not worried about it.
But... 50K... probably due for belts even if they look good the rubber could be hard or the belt could be stretched more than the tensioner can compensate for. I would replace the tensioners as well. They are not very expensive and are easy to replace once you're in there.
