(ET) Transmission noises, odd

[Christopher Zach <cz alembic crystel com>]

So it's spring, which means time to remove the snowblower and hook up 
the tiller. Did some tilling in the yard, the NiCD batteries just hold 
up exceptionally well.

However while tilling I started to hear a grinding sound from the 
motor/transmission. It looks like it is coming from the tranny. I'll try 
jacking it up and replacing the transmission oil, but does anyone else 
have any ideas what could cause rumbling in the transmission, and if it 
can be replaced?

Chris