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I had almost an Identical sound on my 77, 2.7. I would guess that it is valve train related. In order to narrow it down, go and buy a stethascope from harbor freight and just start probing around the intake ports and the exhaust ports while it is running. Also dont forget to probe the injectors. On some of the other cars that I have worked on (non porsche), diesels etc the injectors can get really lound. Anyway after you pinpoint the cylinder that the noise is coming from then it is time to do some light surgery. The first step is to do a valve adjust as loose valves can also make this noise. While you are adjusting you can loosen up the elephant foot to max and slightly wiggle the valve you can also get a small peek at the springs and see if any of them are visible broken. On another posting in this forum one gentleman had a similiar sound and it turned out that his inner spring was broken on a valve. While the elephant foot is loose take a good wiggle on the rocker to check for play between the rocker and shaft. After you button it back up and if it still makes the noise then you may need to remove the rocker from the offending cylinder and pull the valve retainer and spring. Pelican Parts sells a special tool that allows this to be done while the motor is in the car. Warning, you will need to be very patient. I think it took me 20 minutes per retainer just to get them out and that was using magnets, needle nose pliers etc etc. After the valve springs are removed and with the piston at TDC then you can push the valve in until it hits the top of the piston and wiggle it side to side. On my intake valve there was no perceptable wiggle. On my exhaust there was around 1/8" of wiggle and the valve guide seal was completly destroyed, basically 100% missing except for the metal keeper that tightenes it to the valve. Hopefully it will just be a broken spring. A couple of weeks ago when I went on the hunt for my problem a couple of people here on the board said that it was more than likely a worn valve guide and they were right. Some of the other problems that I was concerned about was the possibility of a broken or pulled head stud which would allow the air pressure to escape and make a similar sound. Fortunately it wasnt. Just a worn valve guide. Now I am looking at a complete rebuild due to the weaker case on the 2.7. If I had a 3.0 I would simply do the top end if the valve guides were bad.
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