93 honda rattle noise engine

About 2 months ago, i noticed a rattle noise from the front (L) of the engine. It occurs loud at low RPMs (between 1000 and 2000 and then seems to go away or is not as audible. Occurs when I press the gas pedal; once I reach a cruising speed–fine again. I’ve had it in the shop and they checked the engine head and pan at bottom; used and auto stethoscope to try to find noise–no luck…spent several hundred to no avail. I checked the heat shield–no luck. I think it’s coming from around the alternator–pulleys area; mechanic seems to think it’s the valve plate in the engine itself. Now I’m waiting for it to just break and leave me on the side of the road. Any ideas what can make a metallic rattle noise and have a mechanic stumped. I’m at my end on this one. Haven’t checked motor mounts yet. Any advice, highly appreciated.