British Airways Boeing 747-400 in D-Check

They inspect pretty near every part on a 747 during a D check. Including the toilets. ▶ this video So far as the HH-53 (the USAF version), when I rode one in the 80's, the saying was that if it wasn't dripping hydraulic fluid, there was something wrong.

Another favorite aviation-related one, though more pop-sci-oriented, is this look at a 747 going through a D ("heavy maintenance") check that basically takes most of the plane apart and puts it back together, covering all the bits and how they need to work: ▶ this video

Other factors that make aircraft like this one dirt cheap are that older aircraft (especially the 727) are very loud which makes it difficult to use them in commercial service to areas with noise restrictions. Hush kits are available but they're expensive and don't quiet the aircraft enough to operate some sensitive jurisdictions. If the aircraft is also near the point where it has to undergo a D-check -- the most expensive kind of maintenance check where the aircraft is very nearly completely disassembled, inspected, and re-assembled -- it would make more economic sense to sell it for scrap than it would to continue operation. The BBC has a great documentary on a BA 747-400 D-check that has been posted on Youtube: ▶ this video