"Mourning" sound on 1968 DeVille

Hello everyone,

I drive a ´68 DeVille and I have a problem that I thought someone
may have some advice on...
When driving the car at speeds above 50 mph and taking my foot OFF the gas pedal a sort of "mourning" sound appears.
As soon as I step on the gas again the sound disappears.
Also I can feel slight vibrations at speeds between like 30-50 mph.

I´d be very happy for any advice I can get from you guys.

Kind regards from Stockholm, Sweden.

Stefan