I had a similar issue with a 12 yr old Onkyo AV receiver (5.1). The left channel seemed to always be flaky, regardless of what speaker was connected. Minimum charge to diagnose/repair will likely be at least 1/3 the price of a new rig. YMMV. Luckily, the newer receivers are jam packed with features at a similar price point that you paid 10 yrs ago.
You do have a lot of decent choices though, and can finally get HDMI
I have a Sony 5.1 surround sound receiver . It is about 10 years old . Lately when i play a cd I get some crackling noise i my center speaker. I checked the wire connections they were tight. I used a lens cleaner on my cd player. Some Days it works fine . Then the next day it starts making noise again. When i watch a dvd center speaker is fine . I hooked up a different cd player and still had the noise in the ceneter speaker. All my speakers are new Polk Audio . It was doing it on my old center speaker also.