Suspicious Knocking & Oil Level Reduction

[TemetNosce]

Okay, I have two issues with regard to my '09 Toyota Vitz (2sz-fe) that I've not had for long but was trying to get a better understanding of the issues I'm experiencing.

1) Under load and light > moderate acceleration; usually 0 km/h to 25 km/h AND on motorways when gently accelerating on slight inclines at approx 100km/h the sound occurs.  (I've attached a link to the sound of the noise)

It is a a rattling/ knocking sound that has become quite common.  It goes away briefly when the car has sat for a few days but once warmed up, it makes the sound during most commutes.  The car doesn't physically shake and I cannot physically sense the issue from vibration, only audibly.

2) I was given this car recently from a friend that had not maintained the cat so I know that it sat and hadn't kept up with service for a couple years.    I took it in for a service and mechanic said the oil level was shockingly low.  I've kept a closer eye since then and I feel I'm topping up oil far more than I ever have with any car.  Maybe a quart every month?  (It's the 1.3L engine FYI).  I would think most people would suggest piston bearings or something but I'm not seeing any smoke out of the exhaust or any discolouration in fumes exiting the exhaust.  I do notice the engine chain cover and around spark plugs, exhaust manifold there is a lot of old old crud around where the gaskets would be but I don't see oil dripping on the pavement soanyway I am just trying to get some advice.  

Thanks

Trev