Small Engines (Lawn Mowers, etc.): Scotts 42589x8, transaxel, excessive loading

Question
Just purchased a used 99' scotts 42" cut 16hp mower. (model #42589x8) I understand that Murray is the producer of this mower for Scotts and they are no longer in business.
The First problem is the transmission. My understanding is the previous owner changed the fluid in the transmission and when added he used a heavier oil. The transmission not makes a winding noise when in gear and reverse seems really slow. I know there is no way to check the oil and that you would have open it to check anything at all which it is hydraulic and you would have to remove the air from it. I would not have a clue on how to do this and Im sure I would not have the proper tools. Any ideas on what I should do, I understand a new trans is very expensive.
The second issue is on shut down. When the mower is running and is shut down it make a popping noise that seems to be coming from the fan housing that spins when running. Is this normal or another issue that I may have? Thank you for any help you may be able to offer me.

Answer
John
The first issue with the transaxel I will need to know what the model # is. Here is a generic solution for a transaxel.
I think you have a Peerless #205-024 Hydro Transmission

The purpose of this is to provide useful
information for servicing the Hydro-Gear -1500/
-1700 Integrated Hydrostatic Transaxles (IHT).

If the UNIT IS NOISY the problems may be 1 or more of the following.
Oil level low or contaminated oil
Excessive loading
Brake setting incorrect
Loose parts
Bypass assembly sticking
Air trapped in hydraulic system

UNIT HAS NO/ LOW POWER

Engine RPM low
Control linkage bent or out of adjustment
Brake setting incorrect
Drive belt slipping or pulley damaged
Oil level low or contaminated oil
Excessive loading
Bypass assembly sticking
Air trapped in hydraulic system

The second issue may be the carburetor fuel soilinoid under the fuel float bowl. If there is a soilinoid there make sure the wire is connected and that it works,what this does is when you shut the engine off it stops any fuel going into the carburetor to prevent any back fire or poping sounds.

If you get me the rest of the numbers I can send you a manual for the transaxel and the mower.