Tapping sound only after high speed driving.

[CappyD]

Hi everyone. I have a 2000 QX4 with 144,000 miles.
Very clean, sounds great, and runs great. There were no receipts and I have no past information on this vehicle. I only know an older disabled person owned it before me. That's all I know about it.

If I drive around town staying under 50 mph there is never any noise at all.
When I get on the interstate it drives perfectly for the first few miles. After driving 65-80 mph for 5-6 miles it starts making a ticking sound. At high speeds I can barely hear it and it sounds like a card in bicycle spokes going really fast. The sound never stops on it's own while I'm on the interstate. When I slow down the exit ramp I can hear the ticking slow with the rpm's. When I stop at the end of the exit ramp it continues ticking. When I accelerate I hear the ticking get a lot louder with the rpm's. The ticking sound is loud enough at this point I'd call it a clanking sound. When I speed up to about 30-40 mph and as the rpm's get higher the ticking/clanking noise fades away completely.
If I park at a station before I reach 30-40 mph the clanking noise doesn't stop. If I do not reach 30-40 mph because of traffic the noise does not stop. I must reach 30-40 mph to get the clanking to stop.
After the noise fades away I arrive at work with a completely quiet vehicle. When I start the engine after work it may have a small tick sound but nothing to mention here. It sounds completely fine and normal until I drive 5-6 miles on the interstate again.
I have changed the oil and filter but noticed no change. A few weeks later I added Lucas oil treatment or stabilizer and still no change at all.
4 things I wanted to stress.
1. The sound never occurs as long as I stay 50 mph or below.
2. It runs great all the time. Even while the noise is happening.
3. The symptoms I described in this post happen exactly the same every time I get on the interstate.
4. I have been focusing on the oil but I really have no clue.

I appreciate any help, tips, or advice. Thank you

[macgiver]

mac, gotta differentiate motor vs trans/driveline , although your detailed description is indicativeof valve clearance , or rod/crank bearing , & remotely piston slap or even oil pump. If safe to shift to Neutral while THEN obsreving different mph & rpms' , mix up with breaking and then "engine-breaking" etc. - see if your assessment is still the same , or you have some additional info?
Before you try adding any more "thickening molasses" to oil - observe the dash oil pressure IDIOT LT. as you crank to start the car first time STONE COLD & after LONG LONG "down-time" . like early morning ,2 days no-run.IF LIGHT FLICKERS , and/or doesn't go out quickly(1-2sec.) , and MUST go out & STAY OUT - indicative of main oil "galley" pressure problem , likely causes = main bearings(crank) , rod bearings , oil pump worn or bad pressure relief, chk. balls behind oil filter in the block (if used) could be "rolling around" somewhere BAD .
Keep an eye if ever coolant & oil might have mixed , I know you know your oil ,but check radiator cap in cold engine to see if previous owner had coolant contaminated oil running through the motor which would "fry" bearings.Contamination( I don't have your eng. code) could've occurred where an INTERNAL water pump went completely bad , blown/ leaky gaskets etc. and you MAY SEE REMNANTS of oil, sealer clumps,etc. in the coolant which was hastily changed , very noticeably left ON radiator cap too.
Lastly a direct tap of main galley oil-pressure gauge whilst doing all the above (duct-tape gauge to window & drive ,using 8' hose) note "pulsing" dial pointer=bad ,most importantly note pressure BFORE ,DURING then AFTER noises!