The lid is for scanning slides and Transparent 120 film type stuff. Has
a moving lamp in the lid to pass light through the film.
is this scanner very old ? you may want to lubricate the guide shaft
within the scanner. The thump could be the drive toothed belt jumping a
tooth because the head is sticking on the shaft. page 4 502-head RHS
Belt 532.just needs a drop of oil.(not WD40 as this will dryout)
When you pull it apart which is easy This is for the V700 and its the
dotted part on page. Try not to get any dust onto or into the scanner
you may endup with lines in the scan.
http://www.betterscanning.com/scanning/dismantling.html
Several different models but basically all the same.