NetxTown,Right now, my biggest worry is that I will do the "wait and see" - and end up with a ruined engine. Hence, the stethoscope is exactly what I need to crawl under this pup and start homing in on where the noise is coming from. I question if they dropped the skid plate and probed those areas noted by the transmission shop. I just find it somewhat incredulous that a fully qualified shop could not even come up with a "we think it might be..." after having the truck for 1.5 days....and supposedly spending over 3 hours diagnosing it.
The screwy part is that I am having absolutely zero performance issues. And why do those stupid engine codes not pop up when i need them the most! LOL!
Another Thought is to FIND an Experienced OLDER NISSAN MASTER TECH that has Plenty of Experience with the 4L Frontier. A Sincere Call to the Nissan Service MANAGER @ A Trusted And Recommended Nissan Dealer to get his recommendation Might be a Good Call Too. Not all Nissan Service Department's are the Same. Also if You don't know one,, maybe a Call to Nissan Customer Service listed in Your Owner's Manual Might get some Helpful Advice on Where they recommend to take it to.
I try to do all that I can Myself to save money. I remember when I had a leak on my '93 4 cylinder hardbody. @ first I thought I had an Oil Leak comeing from the Engine, cause it was Oily and Dark on the Garage Floor. I started at the Bottom with a Rag, Shop Light, & a Shop Mirror. I steadily Wiped going UP. I must have crawled under the Truck 22 time's, looking and Wipeing up from the Bottom and then looking Down from the Engine Bay. Finally I tracked it to underneath the Water Pump and useing the Shop Mirror, My Finger, and The Shop Light, I could see the Small Leak coming from the Bottom of the Water Pump. I then ordered the Water Pump from The Dealership and Scheduled for them to replace the Water Pump. I talked with the Technician that did the Replacement and He said, " I Don't know how You were able to find that leak" .
IMO it would be best to make a Serious Effort to find the Noise before a BreakDown Occur's. It might save You a Ton of Money if you can find it before Serious Damage Occur's. I do go by, " If It Ain't Broke Don't Fix It", But if the Writeing is on the Wall then I also don't believe in Tempting Fate Either.
BE EXTREMELY COGNISANT AND CAREFUL USEING THE STETHOSCOPE WHILE THE ENGINE IS RUNNING. You don't want to get in a Hurry and get something entangled in the Moveing Belt, Pulley's, Cog's, and/or Fans's. It's Dangerous so just be Extremely Careful. Remember the Electric Fan Goe's On and Off @ It's Own Time.