For experimental purposes, try a mid grade tank of fuel. Run tank almost empty and try a higher octane. It's worth a shot.I have an 2006 Colorado with a 3.5 liter engine. It has 110k miles. Last year I started hearing a knock coming from the front of the engine. It has gotten a bit worse over the last 6 months. The sound is present with or without the serpentine belt attached. The knock frequency is half the engine RPM at all speeds, cold or hot. You can have a listen to the sound at idle via the attached link.
New Recording 6.m4a
A friend is in the midst of pulling the timing chain but so far has not found any obvious problem. Any ideas?