"buzzing" at base of skull

[Mint boi]

So this is strange. My Ferret has found that the last 3-4 times he has taken 210-250 mg codeine, about 2hrs after dosing, as the peak is wearing off, he had noticed what can only be described as a strange buzzing sensation at the base of his skull.
Its really hard to describe, but he's pretty sure its not just percieved. Its almost like a buzz of electricity, lasts maybe a second, its not a strong sensation, but very noticable, and he's almost sure that he can actually hear this "buzzing" too.
Has anyone else experienced anyhting like this? My Ferret has never noticed it with any other drugs, he has limited experience with other opiates and hasnt noticed it with them.
As I said he's almost sure its not just imagined, but maybe it is. Could have something to do with the CNS effects of codeine, but again never notice with any othe CNS drugs. Its very strange, not unpleasent, just neutral!