So im driving home and as im driving up the street I notice something... myCEL isnt on!!! for the first time since ive bought the truck about 2 mos ago and working on it every day. I was so excited I spilled my drink on the passenger floorboard... okay shortly after I noticed that I came to a stop light and sat there idling and really relieved.. well something starts rattling or banging either in the dash or in the engine compartment somewhere so the first thing I did was turn the AC off and recirulation button and the sound sort of went away, I dont know if it was a coincidence. BTW I am also driving on my first full tank!! Premium dropped to 3.44 here! As I get on the home stretch the truck starts going wacko with rpms going up and down by about 500rpm and the speedo is going haywire and its bucking and jerking. I figure its the VSS?so I will mess with that when it all cools down. Still no CEL at this point well further down the road the CEL comes on again there it is... could it be VSS related?In these past two months I have tracked down every ground wire I have read about, sanded and greased then tightened it back down. All of my lights work fine now due to cleaning sanding and greasing all alternator connections. Is it normal for the CEL to go back off on its own? Last code I got a while back was 0326. I have replaced plugs and wires, cleaned as many sensor connections as I could using contact cleaner then dielectric grease on the connections. Fuel filter is replaced as well. I do have a lifter tapping on the number four cyl or a flat lobe, can burnt or stuck exh valves cause this noise and behavior? Im sorry for the long post but you know how it is when you have worked pretty hard on something and you feel like you are at square one again.

 

A stuck valve could cause this I suppose, replace the VSS and then see what happens. I dont remember if it was '96 or '97 whenthey started using the carbon cutter exhaust valves.
As for the noise in the dash it could be your HVAC blower going south. On GM cars that is how it goes, noisy and then the motor seizes and no more blower. Goos news is though that the blower will make noise like that and still work for like 10,000 years.
Next time it does it rather than turn the A/C off turn the blower to high and see if the noise gets louder or faster.