Ford 640 howl noise in trany or rear end

Thinking of buying a ford 640 tractor with 4 speed trany however while driving wide open throttle in high gear 4th gear when I quickly push throttle to slow rps I hear a howl noise in trany or rear end. The noise only last a few seconds. Could this be 4th gear or rear end going out?
Or could it be old fluid in rear end or trany it was only 45 degrees yesterday when I test drove this tractor. The minimal wear on the peddles of the tractor make it look like a low hour tractor but tachometer is new so don't know hours.
Should I buy it or pass?

Before you do either, why not check the lube level and condition.

The lube in the rear end with dip stick appeared full and not dark in color a bit like honey.

If you checked the oil by pulling out a dip stick you were checking the hydraulic oil level.

This tractor has 3 separate compartments for the transmission; hydraulic system; and rear end.

To check the oil in the trans pull out the square head plug just under the foot step. Oil should be level with bottom of hole.

To check the rear end pull out the square head plug on the rear case right by where the axle housing bolts on. The oil should be level with the bottom of the hole.

The few times I have driven my 641D in high gear on the road at relatively high speed, I thought it made quite a lot of noise. But I very seldom go that fast, and am much more concerned with how things sound in first, second and reverse, the gears I use 95% of the time I am driving the tractor. So how did things sound in the lower gears?

If the dipstick you checked was on the left side of the housing and a little behind the clutch pedal, that is the dipstick for the hydraulic system. My 641D does not have a dipstick for the rear end. There is a plug that can be removed to check that the rear end compartment has enough oil in it, and also there is a fill port like a barrel bung where you can add oil that you could look in. You don"t want the oil either to be low or too full. I have read that it is OK to use the Ford spec hydraulic oil in the rear end, but I choose to use heavier gear lube. It is possible that the rear end oil can get contaminated with water from rain or maybe condensation, so checking the rear end oil is something I do every couple of years--once I got way more than a quart of water out when I drained the rear end. That can not be good for gears and bearings.

Should you or should you not buy that tractor? It is hard to advise someone without a lot more information. If it is supposed to be a perfectly restored machine with a price like a perfectly restored and beautiful machine and that it is something that you might only take to shows, it would be of a much greater concern than a worker tractor for a couple of thousand dollars. Also, could you fix a rear end or transmission yourself, if necessary, or would you need to pay someone else to do the work for you?

Would I buy a tractor that made a worrisome noise? Maybe, for the right money. Every old tractor I have ever owned or operated has made worrisome noises on occasion...but a noise in high gear decelerating quickly from speed would not bother me much, since I just almost never go fast on a tractor. Good luck in your decision, and I hope you make a good choice!