Bubble under shingle roof

I noticed a small elongated/linear bubble under my newly shingled roof, in one spot only. I think it maybe under the 30lb flet paper itself. What is this? What can be done to fix it?
Thanks
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It may eventually go away. Seems to have started to become a problem when they went to fiberglass roofing shingles. They show every problem. I had a roof put up after a tear off and had a bunch but eventually they went away.

roof,
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1. What is causing this bubble? 2. Is it under the felt underlayment or is it the shingle itself?
I am using Certainteed XT-25.
Thanks
Art Begun wrote:

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In my case it was underneath the felt. Not only did shingle material change about 20 years ago, so did the felt. I think the felt does not let air pass thru and so the sun hits it and air expands under it forming a bubble. Just my theory. Some manufacturers of shingles started to recommend no felt be used if code did not require it to avoid the problem but I'd rather have felt with temporary bubble. Of course we are assuming your bump is the same as what I had.

itself.
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I am wondering where this air will go. It is winter now and there is a bubble. I am wondering what will happen in the hot summer days.
When you say eventually they went away.. How many days did it take? Months?
Also did it show up in winter or summer?
Art Begun wrote:

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The air will eventually just pass thru.... just like a balloon eventually looses its air.... same with tires.

shingled
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