Rattling sound under hood at idle and low speed only

Does it sound like a valve or piston, or something loose on the exhaust pipe ?

These engines don't rattle and especially ping unless they are hot. And the sound of fluid thru the heater core. Tells me you are low on coolant. The engine is not cheap to replace.

It could be a leaking hose, and the PS fluid is dropping on the hot exhaust. Check the fluid level in the power steering reservoir. If low or dry, look for a leak.
I don't know about the rattling-better have someone look at it.

I would check exhaust system for loose heat shields or bracket.

Sounds like time for a new engine to me. (severe engine damage due to oil loss or overheating)

Check to see if one of your brackets holding the exhaust pipe has come loose. That'll make a rattling sound - especially if it towards the end by the tailpipe. When you're accelerating - more air is forced thru your exhaust at a faster rate, so it's not moving as much as when you're sitting still and the truck is just at an idle with less air/exhaust moving thru the tailpipe. If it's only making the noise while the truck is stationary & the noise stops one the truck is moving - that pretty much rules out anything to do withe the drive-train or suspension. You are sure the noise is from the back & not coming from under the hood? If under the hood - check your oil level ASAP.

Check for loose heat shields the cover the exhaust. I find this is the source of many rattles. They are made of sheet metal and if they are not clip or tightened down the low vibration frequency when the car is running at slow speed will cause them the hit the exhaust pipe/ front mainfold.

The rattling is a worn-out timing chain (rattling should be louder at one end of the engine). Get it changed now - they will eventually break, closely followed by your bank balance!

when you turn on the ac in your car it slows the engine down that much more and things under the hood will react to this slow engine rpm differance. does the rattle stop when you pick up speed , if it does you may want to get the idle checked on your car some times you can correct this problem with this answer. if your in a real nhot area the ac pump has to word that much harder to compensate for the heat

This is pretty hard to diagnose without being there. I would suggest that you take a hose with ends cut off and listen through it, since you are mechanically inclined. I would also check things that may be rattling against the firewall, the exhaust system for rattles as they tend to do so when bushings are worn out, and recheck the things you have if you haven't done so with hose or stethescope. I had a similar problem that was a loose exhaust bushing... Hope this helps you, Rick