2002 Mercury Cougar, 2.5L V-6 DOHC, 35th anniv edition, 110152 miles. I recently had the transmission repaired (for the 2nd time since I've owned it) and when I went to pick it up I was told the engine was making a knocking noise when they test drove it after the repairs were done. Sure enough it now has a very noticible engine knock. They said it's sounds like a piston rod going out or a bad bearing. To me it sounds like a top end knock and seems to move around to different areas and occasionally the knock sound changes pitch and is a definable top end " metallic" knock. All cylinders are firing, nothing loose external, any possibilities before I tear into it?