Strange shaking phenomenon [solved]

[Pep]

I could use the assistance of a seasoned builder. I'm building a hovercraft and it is exhibiting a strange bouncing phenomenon even when none of the engines are active. This phenomenon only began to appear after merging two layers of the vehicle together.

I believe I have ruled out all piston/mechanical elements as causes.

Update: I'm an idiot. There was a tiny bit of slim invisible material under the vehicle. I should have figured it out but I was very tired at the time and didn't even consider it.