The 'new' converter that came with my rebuilt C4 had one. This converter turned out to be a bad or incorrect balance. I purchased a rebuilt stock type. This one had the plug too. They are very easy to miss, it's going to be between two of the bolt studs.Let us know what you find, I’m very curious. Also, I don’t believe all torque converters have a drain plug. I don’t recall seeing one on my B&M when I installed it but that’s been 13 years ago.
To the OP check out my photo of the blue converter....if you have any photos of your converter bolt mating side go back and look at it for this. On mine you can see it between the studs on the right. I do not remember if I had a rotating interference angle when she was bolted against the plug but it very well could have.
Not trying to send you down a rabbit hole, but this is very easy to check without even unbolting the converter. You'll want to to look on the back side behind where the nut goes on. Look between the studs for something contacting the flexplate. If you do find that, don't panic it's pretty easy to loosen pull 3/4" back and then rotate & reset. You wouldn't even lose tranny fluid. I believe that mine, once all 4 studs were through the holes, the plug could be rotated to one of the larger holes at the same time.
I'll upload the images after this message, can't do it off this machine.
Just an guess but if you have no knock on idle but then it gets all loud on increased RPM then it becomes more likely it's hitting something in the bell housing. Again this is a super easy check compared to tearing down motor and tranny. I had to take my c4 in and out 6 times to test and resolve vibration issues.
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