Hello, how do I get rid of an old riding mower? I thought it just needed a battery but when I tried to charge it, it made a strange noise. Thank you.

Hi,
Often there are lawn mower junkyards around. Best way is to ask a dealer who they wholesale their trade-ins to. The dealer may know of a junkyard, or someone who buys used from him may know. If you are lucky and find a dealer who sells off some or all of their used machines, that person may give you something for your old mower for parts, which is what the junkyards are good for to begin with.
Peace,
Clarke

Sell for parts on Craigslist or give it away free.

Or put it in your yard with a Free sign on it. If it is not too dilapidated looking it should be gone within a day. But depends on how much traffic and type road you live on.
And again, depending on your location, there is most likely a recycling center (junk yard) within a reasonable distance. Some may even come get it. You may even get a few bucks for it.