https://www.audiosciencereview.com/forum/index.php?threads/anthem-mrx-1140-fan-noise.34258/ and other posts on AVS forums and the like https://www.avsforum.com/threads/owners-thread-for-mrx-540-740-1140-receivers.3180183/page-142#post-60894882
They're cheap small computer fans that probably run...