1997 Toyota 4Runner Engine Ticking/Tapping Noise

I believe my serpentine belt was checked during the recent inspection but the spray bottle sounds like any easy way to check. As for the timing belt, I can look into that. I know it was changed around 88,000 miles.

If it helps the noise may better be described as a sewing machine.

I did the water on serpentine belt trick, nothing. I purchased a Mechanics Stethoscope at Harbor Freight for $2.99. The noise made after the car has warmed up and at idle (tapping ticking) is clearly resonated by the fuel injectors, at least the 4 I was able to easily access. Other parts of the engine like the front (timing belt?), Sides (valves?), Top (intake manifold) did not make any noise similar to what I hear.

Assuming it is the injectors and I realize I may be going out on a limb here, what causes noisy injectors? Fixes? Concerns?

Thanks for answering all my questions. Great advice.

Did you ever sort out the "sewing machine" noise problem?
I have the same noise and I'm currently attempting diagnosis.
Like you, the noise is only when hot and the sound is as you describe (good description incidentally!) And comes from top of the motor, drivers side.
I bought this at 140,000. It already had the cam belt, idlers, W-pump, seals all done @ 95,000 miles by the previous owners garage.
I also got the (Dr.) Harbor Freight - stethoscope and got to work listening.
After removing the cam belt top plastic cover, the noisiest place was the top idler bearing - replaced it - no apparent change.
Took off all the ancillary parts - AC compressor, P-Steering pump, alternator and fan and bracket. W-pump is quiet (Aisin brand) but bottom tensioner pulley was noisy - replaced it - no apparent change.
Took off both valve covers and checked shim/camshaft clearances - 1 x exhaust @ 15mil and 2 x 14.5mil otherwise all others in spec.
Scissor gear(s) tension appears good and same as passenger side gears.
Looked down intake ports and could see intake valves and cylinder honing lines when valves open. All looks good and no foreign objects (like a bolt etc.) Rattling around on top of the valve faces.
Am now going to remove camshafts from the drivers side, attempt to lift out 'buckets' and inspect valve springs etc.
A thing I noticed is how clean the drivers side valve-train is compared to the passenger side. A lot more carbon there, maybe 'cos PCV valve 'suck' side?

I'm 98% sure my issue is noisy fuel injectors. It's loudest when the engine is warm and the fuel is cold. These injector have run millions, if not billions of cycles and must be worn to some degree. I think the whole injector is just a little bit loose, especially when the cold fuel keeps the injector internals cooler (cold metals shrink) and the engine keeps the injector external warm/hot (hot metals expand). Nothing wrong with it, just a little bit of play in the injector. I don't have any hard evidence to prove this, but this is the only explanation that makes sense to me.