[ARC5] nasty hum

[J Mcvey via ARC5]

I just recapped a BC453 beacon receiver ( with NOS late issue plastic top "pot caps", which test OK-ceramic?) . 
It works pretty well even before alignment, in fact it's a little better than my Yaesu FT450 on that band. 
I'm using a well worn 28V dynamotor, runnin on a clean 24V supply.When I turn the gain down enough to where there is no receiver audio, the background hum becomes obvious.Its a positive almost triangular looking glitch on the scope. 

Is it the dynamotor or are my NOS caps not so NOS?