This is the damndest thing. 86 firebird, 305 with low miles. Was stored for 10 years. Major service completed. Runs very strong and tight, shifts firm. This knock started a few weeks ago, but never consistent. sometimes its not there at all, sometimes it will start quiet then get louder until i think the motor will blow up, stop.. put the car in park, rev and there is no knock.. then i may hear it slightly again during the trip. Once it did it right after start up.. but only one time. Took the car out the other day for a 7 hour cruise with the family and it never knocked once. It comes and goes. Today i disconnected all belts except alternator and water pump. Took the car out and after about 3-4 minutes it started in but was gone by time we pulled back in a few minutes later. I for the life of me can not figure out what could knock so quietly to violently some times and then be completely gone other times. Have not yet been able to listen under the hood when it was knocking. Pulled into the garage today and was just as quiet and smooth as could be. Any help with this will be appreciated..

Could a knock sensor cause this erratic knocking?

sounds like a lifter. Happens to alot of chevys. they are hydrolic, so its knocking when the lifter is bled down, and not when its not.

The car calls for 5w-30 energy conserving oil. Currently have 10w-40 in it.

Do you think adding a quart of trans fluid might help? Have heard somewhere it can help fix sticky lifters.

There is one of those brands of lifter queter things that actually works pretty well that I've used before, after talking with a guy that had alot of experience with trying different ones. I can't remember which one it is right now, but I'll think about it. Generally when they are that loud, the oil weight wont effect it that much, just maybe a little, I wouldnt put trans fluid there. I used to work in a machine shop/engine shop and never heard of anyone doing that. Maybe someone else knows about that.

Dump a quart of trans fluid in there and Seafoam it. You can get it at almost any auto parts store. Dump it through your brake booster hose while using your throttle bracket to keep it alive then take it for a spin around the block before you change the oil after it's all in there. Change the oil while it's hot to get it all.

I've used this to clean up quite a few motors, most notably a 65 Fury with a 383 big block that sat for 30 years but only had 50k miles on it. It smoked like hell when I seafoamed it but was pretty damn clean inside when I had hardened valve seats put in the heads a couple months later (had to use lead additive before that).

Well i drained some oil and added a quart and a half of transmission fluid. Put around 30 miles on it and she is running quiet so far.

I think the 10w40 may be a bit heavy in this cooler weather so after a couple of hundred miles i'll change it out to 10w30 and may try that sea foam.

Thanks for the input guys!