350Rocket, the cam synchro usually starts out as a chirping like a bad belt, only at the rear (driver's end) instead of the belts. When my synchro failed, it was a grinding sound coming through the firewall on start-up. I shut it down and had it towed. I changed the syncho with OEM. No aftermarket on the synchro's, they're no good with many failing 10k after install.