Two pairs of Garmin Rally XC200 squeaking when pedaling

I'd suggest to grind 0.5 mm with a file from the sole so that the cleats sit deeper in the rubber of the sole.
If you do so, please let us know if it helped.

Ro Lee, you're my hero. You seem to have nailed the problem in your post! Many thanks for this!! While just spraying the lube (Finish Line Pedal and Clear Lubricant) only helpend for about 30km, I'm now for about 200km with strongly reduced squeaking (say down to about 10% of what it was before). Not yet sure what actually did the trick, because it did the following 3 all together:

1. Spraying again, yet exactly to the downside on the back of the pedal (see Ro Lee's picture).
2. Grind somewhat from the sole of the shoes as Ro Lee suggested, yet probably still less than 0.5mm ("I'd suggest to grind 0.5 mm with a file from the sole so that the cleats sit deeper in the rubber of the sole. If you do so, please let us know if it helped."). I must say, it feels strange doing this, so I was careful.
3. Tightened the cleats more to the shoes.

I tested 2 different shoes (both Shimano's though, started with the older pair) on about 100km rides each and the squeaking - finally hallelujah! - was as good as gone, with a little exception left and right sometimes. Finally fellow cyclists didn't look at me as a dinosaur who doesn't maintain his bike properly 

I'll do some further testing, especially to make sure the cleats sit somewhat deeper as Ro Lee suggested, yet, again, so happy that this constant squeaking is as good as gone.

I have Shimano ME7 shoes.  They squeaked twice last year.  I used the silicon dry lube on the pedals and that took care of the issue.  I probably have 60-70 hours of riding on the pedals (Trainer and Bike) and haven't had any issues. 

This is very bad design and Garmin should recognise it. It makes no sense that Shimano's SPD pedals do not squeak at all, whilst those pedals make me look like a bird killer every time I push them down. I've used SH51, SH56 Shimano cleats and the Garmin provided cleats, using a Giro Gauge Off Road shoes. All of them squeak. 

Fixed it!! - An old piece of inner tube placed across the pedal eliminated the squeaks for me!!!

My replacement pair (XC200) under warranty from Garmin developed the same issue - this was a test to try understand what bit actually squeaks - I think it is the cleat on the base/tread plate and this 'bodge' would support that idea although because of the piece of tube it does mean that your cleat is held tightly on the pedal.

Need to try a cleaner solution now to confirm

I had the same annoying chirp.  I installed 2 of the 1mm MTB SPD shims under each cleat...the sound is gone for good.  The cleat needs to press more firmly against the pedal wear plates.  No more squeaks or chirps.

What do these shims look like? Where do they install ? Don’t suppose you could post an image?

They usually are supplied with the cleats.  You can buy them separately however.  Here they are on Amazon: www.amazon.com/.../B01EYECRKS

Not sure why they are called "Road" spacers specifically as they come with the MTB SPD cleat set.