Can't be certain but when an engine suddenly becomes unusually loud the culprit can be the exhaust system. Check for manifold, cracked pipes or a holed muffler.

Your gear box sounds like it's short on fluid, if it does not have enough fluid in it going up hill the fluid goes to the back of the box and the pump is sucking air into the system causing problems, you will need to check the level, you need to get the vehicle level, handbrake on start engine in neutral and undo the level, plug if fluid starts to run out of level, plug the level if OK but if nothing comes out you're short on fluid so you will have to undo the fill plug on top of the box with the engine still running pour in fresh until it starts to run out of the level plug.
When you have one of these units it will pay to drain the unit and fill with Fully synthetic fluid, a lot of people think they can run on dex2 or 3, they can't because they suffer from poor cooling. My advise is to drain the old out and fill with fresh.
There are 2 gearboxes, one just has a drain plug like my modus the other has a dual drainer/drain plug if like me you have only a drain plug look at the bottom of the box you will see a sensor, put a container under it and take it out start the engine in neutral and the fluid will run out.
OK hope that helps!

Oops.. I meant to say level plug. My modus only has the level plug it does not have a drain plug, I wish it did, life would be a lot easier but then it's French!
Opps again the level plug is the drain plug you just need an 8 mm key to put up there and undo the level tube that's up inside !