The 289/302 in my 65 just started doing that a couple weeks ago. It does it mostly, and loudest when cold. I was thinking that the pcv went bad. I plan on picking up a new one, hoping that is all it is. Maybe I should put the vacuum gauge on and see if anything has changed.
65 289 Holley 4160, F4B, Long Tube Headers, 3.25 T-loc, coupe (father, son and daughter project)
64 1/2 coupe 289 4v, D-code (looong term puzzle)
64 1/2 conv 289 4v, D-code (long gone)
68 289 2v sprint 'B' coupe - Bone Stock (Holley 4412, 500 cfm 2v) SOLD, gonna miss that one.