Sounds like valves, I tried an oil additive with a minor change noticed. The sound disappears once I reach highway speed, also it cannot be heard when idling in the driveway. Any ideas?
It may be spark knock aka pinging. can be caused by carbon deposits in the combustion chambers. Try having a de-carb service which is a chemical solution that is inducted into your intake system and cleans the excesive carbon deposits from your combustion chamber as well as your intake valves. even if the noise is still there you will see noticeable improvement.
If you have a 4.3 V6, I have the same thing. I have a 2003 with a 4.3 and it sounds like ratteling in the rockers at low acceleration. Its just the nature of that engine. Nothing to worry about. Keep your oil changed every 3000 miles and it will still be ratteling at 250000 miles.
