Extremely LOUD noise w/Forward gear

My mower is an MTD 13AK608G062 (2000) w/single speed transmission. When I put the tractor in forward gear and drive, I am hearing an very loud noise and am really trying to figure out what it is and where it's coming from. I'm not sure if it's a belt, pulley, brakes, wheels, low/dirty oil or something with the transaxle. It happens in both Forward and Reverse, but it's very quiet to almost non-existent when in Reverse. When in Forward though, it is so loud that I can hear it over the engine and even over the mower deck when engaged. It gets progressively louder when I speed up. Sometimes when going down hill, the noise will get a bit quieter. I already tried messing with the gear shifter while driving to see if maybe pushing it forward, backward, up or down would quiet or even change the noise and it made no difference at all. The noise is exactly the same no matter the terrain.
 
I am just hoping one of you will recognize the sounds/symptoms or at least be able to point me in the right direction, so I don't go wasting a ton of time and money by fixing the wrong parts. Any ideas???


Here's a link to my video where you can hear it... youtu.be/LMumySawkQQ


You'll probably have to copy & paste this into your browser, since it won't let me post clickable links on this forum for some reason.

Thank you very much!

I don't know why the link didn't show up. Try this link to listen and see if it sounds like what you thought...


youtu.be/LMumySawkQQ


You'll probably have to copy & paste this into your browser, since it won't let me post clickable links on this forum for some reason.

Thank you again! 🙂

In case others were needing help with this same noise/problem, I received a very detailed diagnosis and helpful instructions in a response to a Private Message that I sent to AVB earlier today.  To further help others, I have pasted his message in quotes below...
 
 
"That noise is internal to transaxle. It caused by either using too short of belt or the wear of the needle bearings on the input shaft. Both cause the input shaft to tilt and this changes the heel and toe of the bevel gears creating this as the gear teeth try climb out their proper meshing.
 
First I would make sure that the 5/8 x 42 belt is actually 42" and not shorter. There are other setups that use different belts but 42" is specific to your model. Shorter belts will make these transaxles whine as shown in your YouTube video. If not the belt then it will takes pulling the transaxle and having someone to rebuild it, replacing worn parts and shimming as needed. The reason the noise is less in reverse there is less of a load on these gears. The gear ratios are the same it is just the forward gear and shims on the driveshaft has more wear due its increased use. This driveshaft is internal to the transaxle. Also note even after a rebuild some of this noise can still be present but at a much reduced level." - AVB
 
 
… So, from that information, I will be first checking and most likely replacing the v-belt as suggested, even if it is indeed 41.5-42", just because I'm sure it's worn anyway.  If the whining doesn't go away or at least quiet down a bit, then I will pull the transaxle this winter and split it open to clean out all of the grease, check all of the gears and bearings as suggested and replace what I notice is worn or damaged.  The only other thing I'm unsure of is what kind of grease and where to get it that I should pack back in there.  Then, should I seal it back up and if so, then is it a gasket or some liquid sealant that I should use or do I leave it unsealed to allow any excess grease to find it's way out of the transaxle housing?
 
Thank you again, especially to AVB & Roy for all of your help with diagnosing this issue!

As far as the pulley bearings go... wouldn't it make more of a metal rattling sound than you hear in this video if the bearings were on their way out?

I changed all 3 belts with new OEM belts and got the same loud noise. Then 2 days later I noticed it starting to make a clicking or grinding sound when I tried to drive uphill in Forward. I can drive flat, up or downhill in Reverse all day long.  What do you guys think?