First off its time to do a compression test, this will tell you if loosing a piston, rings, or a cylinder. Be happy if your find one low but still with-in 20 PSI this is not too bad on an older motor.
Before you break out the old .45 to put her out of her misery, go ahead and drop the oil pan and check the torque on the crank bolts, also you can also check each cylinder and piston from here with a cut off section of a broom stick. Check to see if the piston moves around the cylinder bore like a loose hockey puck in the corner at a Stanley cup play off game. This will either mean your wrist pin is gone or your crank arm is worn, either or, then its time to break out the old .45 pistol.
Shake and pull on every thing your oily fingers will allow you.
Slip a .001-.003 feeler gage into your bearing journals looking for a spun bearing.
All of this will tell you if its your bottom end, and its cheap to replace the oil pan gasket if its not down there.
All spinning parts should be checks for loose or missing bolts, Pull your serpentine belt and check the water pump, alternator, and power steering pumps for bad bearings.
Last is you didn't say if you had a 5 spd or automatic. if you have a manual transmission I was look into seeing if its in there. Pilot bushing,throw-out bearing?
this is the best way to find a noise without me hearing it