I'm goin' nuts over this....car just rolled over to 240k on my way to work....i heard this rattle sound thought maybe heat shield or something was loose....well after investigation found it's nothing loose....then I took head ff and inspected valves and pistons....no visible signs of valves hitting pistons.....so then I dropped the pan and started checking Rod bearings.....number 1 was in bad shape almost paper thin...so I replaced and checked others...they were all fine....put car back together and.... still knocking....but it only does it at 1500 rpms or higher.....someone please give me some incite to this problem....I would greatly appreciate it... Also while I had head off i reset the valve lash to make double sure that valves didn't hit pistons....