Thanks for the input, fellas.
If I may, let's take the conversation a step further.

Let's say I collapse my antenna and do a range check and find there is interference with the engine running vs. engine off.

What's the first order of business in tracking down the problem?
(as in, what's the most common cause/first place to look?)

By the way, in the back of my Futaba 6exa manual, it says "20-30" paces for a range check with a collapsed antenna.
I step about 2.5 ft per pace, so that's about 50-75 feet.