Cold or warm, makes no difference. Low or high rpm, makes no difference. Intermittent loud "knock" noise coming from very front of intake manifold, immediately below the "firing order" designation on intake manifold. This very location also has a switch, or a solenoid, or a relay, mounted to the very location of this "mystery intermittent knock" noise, but when unplugged, the noise remains. Exactly WHAT this particular switch, or solenoid, or relay DOES is also a mystery to me. Noise seems to be coming from precisely the very front and center of the inside of the intake manifold, per my "mechanic's stethoscope". But it is not from the nearby thermostat, nor the water pump. I've encountered the terms "plenum plate", "plenum gasket", and "purge solenoid". No idea what these terms mean, all I know is when it makes this "knock" noise, it sounds serious. Any ideas, anyone? I'm a disabled vet trying to help out a dear friend whom has just been informed of serious health issues, and he really misses having his beloved truck. Thanks in advance for your responses.

Welcome to DF!

The 3.7 doesn't have a plenum plate that I am aware of, so those are not a concern. Purge solenoid should be separate from the motor as well.... Overhead cam engine, right? I notice the drives for the cams have a fair bit involved. Is it possible the noise is coming from BEHIND the water pump? (that's where three chains come together on various sprockets...)

My first thought was maybe a chunk of something floating in the water pump, but yes ebing overhead cam my next though was a broken chain guide floating and being slapped by the chain, though consistently seems unlikely. Are you saying the knock doesn't speed up or slow down with RPM?

If you could upload a video it would help extremely

Behind the water pump, quite possibly amongst the sprockets, chains, and tensioners related to the OHC! This might be it! This seems to explain the sound, which does not speed up or slow down with RPM. It simply makes the noise about every 5-15 seconds, totally irregularly. I'll try to upload a video tomorrow, thanks so much for the information and also the warm welcome! I'll be sure to let you know if I find anything!

Tried like heck to upload a video of the situation; DodgeForum.com insists my video format does not meet specifications................... Sorry, but I tried, really!
Friend has decided to forward vehicle to nearby repair facility, thanks to all DodgeForum.com members and advisors! You've got a great thing going here!

Usually what works best is to upload a private video to youtube and post the access only link. At least that's how I do it