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If the grinding sound occurs only when shifting into gear, then your 1st gear syncronisers are shot (not uncommon in a high-mileage M46). If the grinding sound is present while accelerating, it's likely that a roller/needle bearing is dying, or possibly some other problem.
The syncronisers are made from brass rings with beveled/pointed teeth on them, and are used to match the gear speeds together before the gears actually engage. They should be considered a wear item, since the brass slowly wears down as it is used. Someone who shifts hard/fast/high-RPM will wear out the syncros much quicker than your typicall grandma driver. Replacing the syncros involves a complete teardown of the transmission, since the syncro rings are set along the main shaft and you'll have to pull the gears off the shaft to replace them.
If it's a bearing going out, this would also require pulling the main shaft and replacing the bad bearing. It can be done by the home mechanic, but not without a lot of time and the Volvo green service manual that covers the M46 rebuild. Hopefully, your transmission does not need a bearing or a syncro, but those would be my best guesses (without actually being there to assess the vehicle).
I wouldn't consider myself an M46 expert, but there's a gentleman on staff at IPD who would definately be able to help you. He's not one of the regular phone operators or desk clerks, but works back in the office somewhere. I was transfered to him after talking with two other IPD techs who attempted to tell me that my "OD-swap" should work, and with me trying to explain the shaft and spline differences. I don't remember his name, but after being transfered to him, I was astounded at the information he could recall without looking at a service manual. It seemed like he had complete exploded diagrams of the M46 trannies memorized in his frontal lobe, and was describing from memory some of the inner parts of the trannies that I was having difficulty seeing (with the units right in front of me). After about 15 minutes of explaining the work in front of me, he managed to talk me out of swapping the M46 mainshafts. -and probably saved me about a weekend's worth of time.
You didn't mention what seal was leaking on the M46. The tranny rear tailshaft seal can be changed with the unit still on the car, the front input shaft seal would require pulling the tranny. I don't know if the tailshaft has a replaceable bushing or not (the M47 5-speed does not, but the AW-70 does), but I would advise replacing it if you are changing out the seal (or you have any noticeable vertical play in the tailshaft).
God bless,
Fitz Fitzgerald.
--
'87 Blue 240 Wagon, 255k miles.
'88 Black 780, PRV-6, 147k miles.
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