The line appears to be a text wrap applied to the top of frame only. You can confirm whether a text wrap is applied by moving another text frame next to it, and see if text is moved away from the wrap.
If padding within a text frame is what you want, then right mouse click the text frame, to bring up the Text Frame Options, and apply an Inset Spacing value.
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Uwe is correct, its the top text wrap value for that text frame. If you are encountering this on all of the frames that you draw, the issue is that the top text wrap is applied as a default in InDesign. To fix, make sure that nothing in your document is selected, and set the text wrap to none. Now it will not be applied to new objects that you draw.
And: If that will happen with every text frame you are adding to a page, you somehow set text wrap as preference for everything, because you invoked the functionality without selecting anything.