Car making ticking noise out of nowhere?

I have a 1991 Ford Ranger and I've never had a problem with it. Right after an oil change a few days ago it started making a ticking noise. I've had some people tell me it could be an exhaust leak and I've had some people tell me it could be lifters tapping (god I hope not). The sound can only be heard when in park. For some reason even when I'm idling in Drive I can't hear it at all. When in park it's loud. Also, It's much louder from underneath the truck towards the front close to where the exhaust manifold comes out and connects with the pipe. Can anyone give me some ideas or tips on how to diagnose this issue? Aslo, what are the differences in exhaust leak noises and lifter noises. I've heard it could be the oil pump too but i don't know.

Really can't give a definative answer due to not being at the vehicle. But what you describe is typical of a load bearing component. Like a wrist pin knock. Lifters will make more noise at first cold start. That's more like a tick. Wrist pins at the onset, won't exhibit noise when loaded. Such as in gear. But will make noise decelerating in gear. Later, knocks all the time. Rods mains will do that too. Crank mains *tend* to knock all the time.
thing is, that they can actually exhibit noise when ever they want. Just the examples I gave were typical. Not set in stone.
if you could upload a sound bite on say youtube and post a link here, myself, and many other seasoned mechanics need only to here the sound and we can tell if its a lifter, wrist pin or rod/crank main. Make sure you record in park, in gear, and a slight rap of the throttle in neutral.
what you can do is use a section of hose. Garden, oxy, any hose. Put to ear and engine.
hose touching the rocker covers is obviously a valve train issue. Weather that be lifter, push rod, valve stem, or rocker.
touching hose to side of block just below the heads is a wrist pin issue.
touching hose to girdle, just above the oil pan would be a rod or crank main.

going from front to back along the block, both banks if app, one might narrow it down to one or two cylinders.

Chances are you got the wrong viscosity when the engine oil was replaced.
just some info. I've seen small shops use the same grade for every car that drives in, no matter if its 10W30, 20W50… Evryone got the same.
I'm putting my best of guess into a different viscossity engine oil.
IF you decided to add one of those engine cleaning product chances are some gum or varnish is rectricting flow to the oil pump(dirty strainer)

Buy an automotive stethoscope and use the probe and the tube to find the source.