100% on what WoW said. The "taps" are not the same. I do not see much of a difference even with the taps being different.

Better? Same taps, very different wobbles: https://youtu.be/izShipRlDbU?feature=shared

That's certainly closer to uniform. Not sure to be perfectly frank. Glass is a highly viscous fluid...so certainly subject to variability in this case.

I would ask for the build sheet and perhaps email the Andersen service line. I have found them to be very helpful in the past.

While they both look and sound more or less normal, they are close but not identical and that's interesting.

I agree with WoW and MWM that the difference in the first video was more pronounced primarily due to how they were tapped, but in the second video where the taps were more consistent any difference was much less pronounced.

Glass thickness, size, and aspect ratio can all make a difference in reverberation both visibly and acoustically. Since Andersen 100 series use single strength glass, that could make them more prone to a slight wobble when compared to an IG using thicker glass.

- disclaimer - I have been out of the loop for several years so it's possible that 100 series might now offer other options besides single strength. But I would be very surprised if they did. -

Additionally, larger size combined with a close to square aspect, even with thicker glass, could definitely contribute to the glass having a jello-jiggle when tapped.

While I see no reason to be concerned from a safety or longevity standpoint, based on the videos, your curiosity is understandable. Honestly there is a tickle in the back of my brain telling me that I am forgetting something that I used to know and should be remembering about this particular phenomenon....

You just got as good an answer you will get out of Andersen corporate.

Note they were identical when they were installed ~6 months ago. Now there's a clear difference as confirmed by two on site contractors...

"Identical" as in confirmed by the build sheet or by a contactor looking at them and tapping his finger on them?

The only thing that I can think of is that the seal may be failing and in a weird way, maybe creating some negative pressure in the process. If that is the case, that's fine as it will manifest as a seal failure at some point in the not to distant future and should qualify for a warranty replacement.

What did you contractor do to "confirm" the difference? Regardless of "feel", its about what you can legitimately demonstrate to the manufacturer. If the build sheet shows the same strength of glass (answered by Oberonn above) and specifications on the unit...what is the provable and causal reason for the percieved difference? The unts are the same size?

There is a phenomenon that can occur in tempered glass with a square or close to square aspect ratio called "oil canning". The term oil canning is used for tempered glass that is slightly bowed (concave/convex) in the center of the lite, but not at the edges. It's almost like the center of the glass is too big for the edges.

If you hold a tempered lite that has this defect so that the convex side is facing you and then push the glass in the center, you will pop the convex to the other side and the side closest to you is now concave. Rocking the glass back and forth will cause the concave/convex bow to swap sides often with a very audible pop each time it happens. Essentially, it wobbles when pushed.

While this peculiarity only occurs in square or nearly square tempered glass and can be fixed by simply retempering the lite, tempered glass with oil canning is really easy to spot and demonstrate (play with) when it does occur, so it should never leave the factory in that condition. A homeowner should never experience it in their windows.

So the reason why I brought up oil canning even though that's not quite what you are seeing is that it did enter my mind when watching your videos over and over, however I dismissed it because what you are seeing isn't quite the same thing, plus the impression that I had from the videos was of thinner glass....and here was where my brain took a short vacation.

I said in my original reply that 100 series uses single strength glass and that is true. I also said that I would be surprised if they ever used thicker glass, and that was when my brain was off doing something else. Of course they use thicker glass when required. For example, while single strength glass is 2.2mm or 2.3mm, glass has to be at least 3.0mm (3.1 commonly used) to be tempered - this is physics, not manufacturer choice - and certainly there will be times when 100 windows will be used for applications requiring tempered glass, therefore 3.0 or double strength glass. So my apologies for any confusion there.

I ended up calling a very good friend of mine, who also happens to be an IG specialist, ad asked her for her opinion on what you are seeing.

We had a great conversation and she said that in her experience what it sounds like is that the sash is "too tight" (hard to specifically define) for the IGU. This is not a diagnosis since she would need to physically examine the sash before making a call, but it is something that she has seen in the past. Her comment on too tight sash was why I included the Cliff-notes dissertation on oil canning to start when it appears that the glass is too big in the middle versus at the edges.

Imagine a frame that can be very slightly stretched. Imagine attaching a sheet of clear plastic wrap tightly to the frame with no wrinkles. It's tight, looks and feels perfect. Now imagine if that frame relaxes just a bit... Pretty sure that you can see where I am going with this.

Disclaimer - Okay, this really isn't much more than a wild-a$$ guess, based on professional experience, but not involving a post-analysis by actually touching the window in question. In my opinion, the thinner glass, being thinner, is a bit less structural than thicker glass would be so possibly more prone to a bit of wobble in the right conditions. But again, this is not definitive in any way, really just speculation.

We did both agree that we didn't see it as a reason to be concerned about safety or longevity, nor did we see it as a specific flaw in either glass or window, but it is out of the ordinary and I can understand if you feel that you need to do more follow up with AW or elsewhere, and honestly I would be very curious to see what other possibilities might be presented.

Oh, and if it turns out that those two windows do have tempered glass, and I really don't think that they do, then forget everything that I said here after the first four paragraphs.

One last question, any chance that the window on the left is facing west while the window on the right facing north?

Wow, thank you so much for the thoughtful response. So kind of you. The window on the left is north and right is east.