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Discussion Starter #1 (Edited)
What is this knocking sound!

Been cruising this site for a while now acquiring any information that I can use as I go along. This is my first thread due to not seeing anything relating to this particular problem. I have an '84 Bronco with a 4 barrel carbed inline 300 six with a C-6 trannie. I've been driving it for about a year now and have always had this problem-more annoying than anything I think. To clarify, the single knock can be heard when slowing down to either turn or stop. As best as I can describe, the single knock sound is occurring at the same engine speed, which is close to idle, and wondering if it has anything to do with the torque converter lock-up/stall speed. Could it be a loose flywheel bolt or loose torque converter-if that's possible? As for the sound, occassionally when I first start the engine and the fast idle doesn't kick in, the engine will knock-diesel until oil pressure comes up, I am assuming oil pressure, because it is non-existent during normal driving. Even when fast idle does kick in, the knock turns into a rattling knock until pressure comes in. This may be a different problem(oil pump,oil viscosity/bearing) but the single knock sound on decelleration is very similar in sound, but the two may not be related. I mentioned loose flywheel/bolt because a friend of mine that has more knowedge than me suggested it. I did somewhat check the flywheel while I was replacing the starter and I did not see anything out of the ordinary from that vantage point.
 

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Discussion Starter #2
BUMP
Does this sound like to anyone that it could most likely be a cracked flex plate? Engine/transmission sounds fine during normal driving. Just a little rattley on start-up and on high revs and the slight knock when decellerating to near idle speed.
 

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MARBLE GARGLER!
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could be a cracked flex plate, ive seen that before. But more than likely some thicker oil may bring oil pressure up stopping the knock. What weight oil are you using now ?
 

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Discussion Starter #4
I finally got a bite on this one, thanks for the reply. I just switched to 20W/50 due to the recommendations on this site due to the older engine. It seems the duration of the knocking is a little less with the thicker oil, but still think it's the transmission flex plate. If it idles at just the right speed, it will not knock near as long-only a few swconds. It does seem to diminish when oil pressure comes up and normal diving is fine, but some of the noise is still there on high rev and slowing down to idle speed. I also bought a Motor Craft oil filter per the site here to replace my FRAM, but I have not put it on yet.
 
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