Sounds like mains to me. The knock might be a lifter, unless it's coming from under the engine. You can replace your main bearings alone, but that is not a good idea. I replaced rod bearings and a couple weeks later the mains starting knocking. I would take it and get an opinion from a mechanic or two. Machine shop here is about $600 to do a long block, if you do the rest yourself.(ex. pull it,strip it, put all back together)
When you find out remember to come back to CHR and let us know what you found.