Transmission problem? I hope not

My 2002 manual transmission Dodge Ram 1500 pickup has been generating a very slight rattling sound (barely audible compared to radio volume set at low) only when I am accelerating in 5th gear. I have accelerated to similar rpm and speed in 4th without producing the same sound. The sound is so slight that I first thought it was static on the radio, and, once I realized it continued with the radio off, I thought perhaps it was a failed weather seal causing an air-related noise at a high speeds. 4th gear testing that I metnioned earlier ruled out the latter idea. The noise occurs primarily around 50 mph, and it does go away if I take my foot off of the accelerator (however, I don’t know if I’ve tested to see if there’s a difference when I have the clutch depressed or when still in gear). Also, I really seem to notice this in the mornings, and I think it’s only started since this last Fall/Winter. So, maybe just a cold-weather thing? Any ideas or thoughts would be appreciated. I’m planning on taking it in for a 70,000 mile service soon, but I wanted to make sure that I shouldn’t go directly to a transmission specialist sooner.

It sounds to me that it is pinging while straining in 5th gear, probably should shift down to 4th until you are going faster.

I agree. It’s engine ping. 5th gear is for cruising on level highways at highway speeds.

I can handle that. This is my first standard, and I’ve kept it well maintained for the last five years. So, I’d be grateful that this might just be my own ignorance coming into play, and I can cheaply remedy that! Thanks for the comments.