1968 Mustang Squeaking But From Where?

Since you say it is when the temp is 180 deg, that is when the engine oil is the thinnest. What you need to do is use a stethoscope and listen to various areas on the engine to see if you can pinpoint the area where it is coming from, if you do not have a stethoscope, you can use a large long screwdriver by wrapping your hand around the end of the handle so that your forefinger and thumb forms a ring and the rest of your hand makes like a cup, and hold it to your ear tightly, just be careful as the engine is running. I hold the blade of the screwdriver where I want to listen, with my hand around the handle, and then place my ear on my hand. It actually works pretty good, it may not be quite as sensitive as a stethoscope, but it will give results. It could also be bearings in the water pump, alternator, power steering pump, idler pulley, etc. You could also get the engine to operating temp where it is squeaking, and remove your serpentine belt, this will tell if it is internal or external to the engine. Post your results. Good Luck.

when i hear weird noises under the hood, i take all the belts off to see if its in an external thing or inside the engine. Last time there were no noises with no belts - i learned the alternator bearings were going bad. Lucky i caught it before it quit working and left me broken down on side of road.

Edit: Sorry rex, i guess i didn't read your post all the way to see you wrote the same as i did.

I will give it a listen and see what I hear. Thank you for the info. I'll keep you posted.

you can pick up a mechanic's stethoscope up at Harbor Freight for $4

Mechanic's Stethoscope