Makes a low moaning or groaning noise

07 pontiac vibe 27,000 miles - when letting all the way up on gas peddle. Seems to be getting louder the last few weeks.

The noise started at about 10,000 miles. Car runs fine and auto trans shifts perfectly. The noise seems to have gotten louder of late. One has to have the radio off and driving on smooth road surface to hear it clearly. This one has me scratching my head. Would love to hear what other vibe owners have found out about this problem. Thanks for any info.