The whine comes from a hydraulic fluid pump and resonates through transmission metal and ROPS (or cab) act kind of like a speaker to amplify it. The L series is intended to be 'simple machines without bells and whistles' and that's actually a reason many people buy them. However this means Kubota didn't add extra fancy stuff to quiet them down (unlike grand L tractors).
I personally DONT like the whine as it's high pitch and makes it past my noise canceling earbuds easy. There is an upside though...the noise lets you hear what strain the tractor is under and basically teaches you to run high RPMs mostly.
If you're getting 'increasing whine' it's because the tractor is under more load (has more weight on it) or something like brakes are dragging. Or you're running lower RPMs than before. Or it's colder outside. Some people said they get more whine by having low fluid level... I kinda doubt it, though maybe this would let sound resonate more since there is less mass in the body.
Regarding 'change the fluid to super UDT2' statements. It doesn't help and actually makes it worse. At 50 hrs, I changed all fluids and filters. On many Kubota models, this is the default maintenance, however on L series they say you can wait till 200 hrs to change it (along other fluids). There is like 7 gallons of expensive hydraulic oil to replace and the fluid looked fine honestly... felt I just wasted time/money replacing it at 50. There were some metal shavings on the filter magnets.
What I find is the tractor with new fluid has considerable MORE whine. Operating at low RPMs and cold temps make it worse.
The whine reduces after about 50 hours of decent usage to 'break in' after fluid change. Once the fluid warms up, it is noticeably less whiny. I always operate the tractor between PTO RPM and WOT. I'd say 95% of the time, I'm at WOT. There is a noticeable difference when its warm or hot outside... the whine is less. Worst situation for whine: freezing temps out, just started up, backhoe/loader is on, 4WD engaged, and going up hill in M gear. Try same thing when it's nice out, no loader/backhoe, flat land, L gear, 2WD, and then it's quiet.
Here is a tip: What I discovered is if you run it a tick under WOT, the overall noise from the tractor (including motor and pump whine) is less, but power isn't too much less. You can play with the RPMs to find different frequencies the whine will resonate at.
All HST equipment whines to some extent, but the higher end stuff less so. My 2 cents is the L series is a nice tractor at a nice price, but it's not in the 'high end' category. Thus if you want a quiet tractor, you're going to trade grand L. I don't think the BX, B, or LX is any less whiney. MX not sure.
