I recently made a circuit thing, it worked great in my "lab" environment but once I put it in the car, it made this high freq ringing, extremely annoying and took it out. In the mean time I am trying to figure out what I 'left' out of the build.

I would say yes to the capacitors... this I noticed from looking at other schematics and re-reading what I should have already known about it.

I am curious to see what advice is given from the seasoned vets, /subscribed