Hello Patrick, Here's another aspect of "false" string. A single string can sound with a beat. I have found this problem mostly in older pianos. Though I did find it in a model D last year. The piano which particularly seems prone to this problem is the older Bechstein grand. It is more often to be found in a grand than in an upright. My solution has always been to take a screw-driver and to place the business end ON the string right at the bridge-pin. Then witrh the heel of the hand to sharply hit the end of the handle thus ensuring the string is seated firmly and making contact with the wood of the bridge. I am 90%
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