Nickel Ping Test Short "Ping" Vs "Thunk"

I have some nickles from the 70's and when I flick them in the air with my thumbnail some have a slight "ping" to them and a few others "thunk". All the ones that thunk happen to be from the Denver mint, but there are also some from the same mint that have the expected small ping. The ones that "thunk" have absolutely no sound at all to them.

The "ping" is not long like a silver coin and all the current-day ones I have make the same quick slight "ping"

From what I found, all nickels, with the exception of silver nickels, are made out of 75% copper and 25% nickel. Have any of you experienced this?

Here are two 1975-D nickels. The one on the right has the slight "ping", the one on the left gives a "thunk".




Her are all the nickels I found that "thunk"