The internal thermal noise is also called 'shot' or 'poisson' noise. https://en.wikipedia.org/wiki/Shot_noise
It is basically electron movement. High-velocity electrons hitting a plate makes noise.
Older FM circuits were crude and rudimentary for the most part compared to today. Newer designs and features for FM AGC, traps, limiters, discriminators and muting have improved FM radios vastly.
It is rare to hear multi-path flutter or VHF images on a newer FM radio. Back in the '60's and '70's that was considered normal, even on a quality car radio.