The 99s and 00s have a Technical Service Bulletin out. The PCV allows too much oil into the intake, and you'll get spark knock, more noticable under acceleration. The fix includes cleaning the carbon out and replacing the driver's side valve cover with the 01+model. If you haven't done that, give your local dealer a call, and ask to have it done.
Another message board had a post about the TSB and said the valve cover was pretty easy to replace. I called our local Ford dealer and they had the part in stock - it was about $40. Here's the message board link:
Boy, I'm glad someone started this thread! I purchased my 2000 in June with 30k miles on it. Since then, it has started to develop an acceleration knock. Higher octane gas seems to help, but I thought it strange that an engine with so few miles would knock on 87 octane. This may be my problem.
I have now tried this fix. It worked for about 60 miles, then the pinging came back.
Bronco Joe said:
Another message board had a post about the TSB and said the valve cover was pretty easy to replace. I called our local Ford dealer and they had the part in stock - it was about $40. Here's the message board link:
Talked to a friend of mine at Ford and he did some checking and confirmed this is a known issue. The part that REALLY needs to be replaced is the baffle inside the valve cover that prevents the oil from getting into the PCV tube to start with. I think the only way to get it through buying the cover at the dealer, though.