How can I get PNG exports to crop automatically like it used to in Illustrator?

[#1]

I use Illustrator as part of my process to create instructional documents, and I am experiencing issues with exporting the file as a PNG that I add to InDesign documents.

In the past I was able to import a PDF from a file created in AutoCad, do final adjustments in Illustrator, and export the file as PNG to use in InDesign.  While exporting to PNG, the file would be automatically cropped to the edges of the image. This made adding and fitting the file into InDesign reasonably easy and efficient.  Now when I go to export the files, the format of the original document is retained.  I don't believe I have changed any settings, and I am not checking the Artboard option. What am I missing? It is common to have over 30 images in a document, so this is really slowing things down.

Thanks!

[#2]

I use Illustrator as part of my process to create instructional documents, and I am experiencing issues with exporting the file as a PNG that I add to InDesign documents.

In the past I was able to import a PDF from a file created in AutoCad, do final adjustments in Illustrator, and export the file as PNG to use in InDesign.  While exporting to PNG, the file would be automatically cropped to the edges of the image. This made adding and fitting the file into InDesign reasonably easy and efficient.  Now when I go to export the files, the format of the original document is retained.  I don't believe I have changed any settings, and I am not checking the Artboard option. What am I missing? It is common to have over 30 images in a document, so this is really slowing things down.

Thanks!

[#3]

I'm afraid I don't work in CAD, but is it possible the CAD documents are exporting a white, solid background? Have you checked your layers to ensure that's not in there? 
Have you tried creating something from scratch in Illustrator and exporting to PNG, to troubleshoot whether the issue is in your Illustrator setup, or your import/export process? Please update on this.