Tensioner usually makes more of a grinding sound... bad bearing. Water pump makes a similar sound (grinding) sometimes but the most common problem is the seal leaks. That's why you replace it when doing a t-belt. You'd most likely head a rod knock WORSEN on acceleration/load if that was the case. You sure it's not valves (possible worn adjusters or valve stems)? You can hold a long screwdriver to your ear and place it against various spots on the engine to try and isolate the sound. Is the sound better, worse or the same on a hot engine vs a cold engine?

My '91 is tight as a drum but it still makes some interesting (faint) noises at idle...... for what that's worth.