I remember a lot of old transformers, particularly the power transformers on the old TV sets, were coated with tar. It was not a real problem, but was kind of ugly, particularly when the wires had tar all over them.
What was the purpose for the tar? My guess was to eliminate chattering of the steel laminations. Yet, it seems the practice of using tar vanished for the most part in the 70s and later. Yet I have not seen any untarred modern transformers chatter......
I will add to this, that the most chattering I have heard from transformers are the small ones which do NOT have bolts going thru the core, such as filament transformers and doorbell transformers. Several times over the years I have had people ask me why there is a noisy sound in their basement and it never fails, it's the doorbell transformer.
This could often be fixed by squeezing the outer metal piece against the laminations, and applying something like laquer or silicone caulk. However, for the small price they cost, I usually just replaced them with a new one.