Some electric fences have a pulsating tick sound to them.  Maybe one of the neighbors by chance?  Just a thought.

I use the Scorpion SA680S just behind the cab of my pick up and a steel mount on the bed rail which is sandwiched between the fiberglass canopy and bolted inside the bed.  The bracket didn't flex a bit but the bed rail did just a tad.  I fabricated a simple angled mild steel secondary support bracket that bolts though my fiberglass canopy.  Being the canopy moves with the bed anyway it's been quite stable.  I just insulated the mount from the antenna and used a large hose clamp to secure it to the bracket.  The bracket doesn't have to be big, just not flex.  My first attempt at a secondary support bracket I fabbed up a heavy piece of HDPE which worked fine but I didn't like how it looked and don't think it was as strong as what I have now.  Both work fine though.  The steel bracket turned out allot cleaner looking and didn't affect performance whatsoever that I can tell.

Best of luck.

Scorpion screwdrivers rock!  I really like mine and wish I'd have bought that from the beginning.  I certainly would have been money ahead.  I'm sure you'll be happy with yours, too.  Mine is the S680S which is mounted similar to yours except I have a fiberglass canopy on my pick up.  The bed rail was flexing a tad so I added a secondary support bracket off the antenna (image on QRZ).  Works great.  I've been running it this way with the 102" SS whip on top for quite awhile now without a hitch.  I'm sure you'll want to experiment some with whip configurations too (eventually).  The shorter whip provided with the antenna worked okay, but not near as well as other configurations I came up with (i.e. cap hats, longer whips, etc.).  Cap hat configurations never performed as well as the good ol' 102" whip in my installation though so I've stuck with the 102". 

Look forward to hearing how well it works for you.  It should really shine on 20m-40m.

Best of luck.

Just a thought to consider, but a friend of mine who runs the Sidekick shared with me he has to clean the coil / finger stock (spring?) connections every month or two.  Between the moisture in the air and heavy road spray apparently his collects more than it's share of road grime causing poor contact between the two.  Not sure if that's a possibility with yours or not...but I wouldn't think it would be hard to pop the cover and wipe down the coil just to see?  It would certainly explain the standing wave you are seeing.  Just a thought.

Best of luck.