Does it do it, in Neutral, as well?
If not, can you put the 1-2-3-4 gear lever in between 2 & 3, so it acts as a neutral, but have the range transmission in high, and see if it will still clunk while standing still.
If it isn't too bad, I'd almost guess, it just needs an adjustment. There are two main adjustments on these tractors.
First, attatched to the clutch linkage, is a little rod, that goes to a 'flipper' on the TA valve body. This 'flipper' should pull a plunger out of the valve body about 3/8 of an inch, with the clutch pedal pressed fully down. I know that 3/8 is the clearance for a 1066, and am assuming a 1466 should be about the same. The easiest way to check, is to have someone press the clutch all the way down, and see if you can just slip the end of a 3/8 inch drive socket extension behind the flipper, where the plunger comes out of the valve body. If it is not 3/8 inch, I'd call to verify the clearance before I'd change it, just to be safe. However, if it is way off, it can cause a clunk.
Another possible source of the clunk, would be the TA brake being too loose. This is the lever that comes off the clutch linkage, that goes into the side of the tranny housing, near the bottom. When the brake wears down, or the linkage gets too much slop in it, the brake does not work, and it allows the TA basket to keep spinning long after the clutch is held down, and it sometimes stops with a clunk, especailly if there is a little wear in it.
You want it tight enough, to slow the TA basket so it doesn't grind much, going into gear, but not so tight, that it won't 'let go' by lifting the clutch pedal up off the floor a couple inches. Usually, it is the linkage itself that gets worn out before the brake pad.
That is about all you can check out yourself, without things like pressure guages and such. Hopefully, it is just something a little out of adjustment due to linkage wear. If not, you may want to get the line pressure checked, because if there isn't enough hydraulic pressure getting to it, things can wear out fairly fast.
Lastly, if the pressures are good, and the linkages are in spec, it might just be the sprag wearing out. Ond of the main causes of this, is using the TA to de-accelerate. My dad used to do that a lot, as he was approaching a driveway or coming into the yard, he would pull the TA back, to help slow things down. The TA in those tractors is not designed to do that. They are designed to help pull you through a tough spot in the field, not to help slow the tractor down. In Dad's mind, he was saving wear and tear on the brakes. In reality, he was beating up the sprag in the TA.
Brakes pads can be replaced, in about an hour, yourself, in the middle of your yard, with only 2 wrehcnes for about $200
The TA requires the tractor to be split in half, and will run $2000+
If you are pulling the TA back, to help slow the tractor down, you will be doing yourself a favor, to use the brakes instead.
My brother learned this in Tech School. He has a 986, with well over 9,000 hours, and the TA has never been touched. He only does two things differently than most guys who replace the TAs fairly regularly.
1) he does not pull the TA back, to slow the tractor.
2) he changes the hydraulic filter, every time he changes the engine oil.
In fact, since we started doing those two things, we have not replaced a TA in anything, and we run a 656, 685, 766, 986, and 1066. The last TA job we had done, was on Dad's 766, about 20 years ago.
