Is the pulse hose cracked or leaking? If not, the float may be stuck closed. Possibly a "tap" with a plastic handle screw driver might dislodge it. There's also a chance you have a piece of "gunk" physically blocking the needle & seat assembly.

http://www.searspartsdirect.com/partsdirect/getSubComp.pd?modelNumber=917273404&productCategoryId=1509200&brandId=0247&brandDescription=CRAFTSMAN&modelName=LAWN-TRACTOR&diagramPageId=00001&componentDescription=PEERLESS%20TRANSAXLE%20-%202000-006A&documentId=P0107012&blt=06 If that picture comes up, you'll see the gears in your transmission. You'll see that some of the gears meet at a 90 degree angle. That's probably where your problem is; they are no longer meeting. You can probably repair these yourself, but it's time consuming.

According to the parts list, the engine has 2 "modules" aka coils. Apparently the kill wire to one has been disconnected??

I have a YS4500 I bought last year and mine makes the same noise when starting out. I'm guessing if I had hilly terrain to mow I would hear it going up hills also. I think you will have to decide how much of an incline your using it on and if your doing a lot of that stuff then it may not last. But as a side note I have a 1976 Bolens H16 with who knows how many hours on it. It has an old Eaton hydro in it and has whined since the day I brought it home. Last time I tried it it was still going strong. I also have a old Ford 18 horse lawn tractor made about 1991 and it whines like that also. I have no idea if the tranny has ever been worked on or replaced though. So I'd say it's hard to say how long it will last. My new X465 Deere makes no noise at all but then that thing is a buldozer compared to my other tractors. Bill