If it's still running it's not a timing chain failure. If it really sounds like a roller coaster going up a lift hill it could definitely be rod knock.
Sound like this?
https://www.youtube.com/watch?v=pYleKxjl0uY
If it does it's rod knock. Stop driving it immediately to salvage what you can. At the least they'll have to pull the engine and remove the oil pan, if it's not too horrible new bearings may cure it but you won't know until then.
Did it happen all of a sudden? Usually a result of oil starvation and high temps (racing the engine). If you ran it out of oil, though, it'd kill it right quick.
Edit: Dudes at Firestone should know what rod knock sounds like so maybe not, then again they're just dudes at Firestone so who knows.