Volvo penta 2040 gearbox droning sound while sailing

We have a 1997 Beneteau 36cc s

We have a 1997 Beneteau 36cc sailboat that we bought new at the Southampton Boat Show with a Volvo Penta 2040 diesel inboard engine and a fixed 15" propellor. We sail with a free spinning prop with gearbox in neutral. Last Thursday a loud 'droning' sound started to come from the gear box which was hot to touch. We recently fitted a new propellor shaft seal and cutlass bearing and the shaft is stiffer to rotate by hand than before. We have greased and vented the shaft seal. Any thoughts please? We live and sail in Qatar in the Middle East.

"G'day James,

"G'day James,
A couple of things to check[do]
#1 Burp seal volvo types drone when running hot-dry.
#2 Relationship between remote gear lever and lever on gearbox; clutch may be slightly engaged.
#3 Gbox should be put astern when sailing.
#4 Check oil level & grade.
#5 Disconnect shaft coupling -eliminate Gbox.
Good luck, hope it is not a Gbox bearing problem. Shakey."