My '96 'trep just ticked over the 96k mile mark, and one morning last week I started it up before work and hear this .. knock knock knock sound for about a second and then it went away.. (when the oil pressure went up). first I thought it could have been a lifter like so many cars sound like, but mine can be heard at some specific RPM's.. like... 2600 RPM. HWY speeds. :| anyone else have experiance with this? how much it costs to fix?