[ctesla]

Welcome to the Automotive Forums;
first, hopefully, that audio was only when oil was low, and now it is performing regularly; and more silent.. usually if that type of 'clatter' is happening internally it is something to be concerned about..
if this noise is still occuring, it would be pertinent to get it to another shop (via tow, not driving the 20mi/30km).. but it was odd that after start/stop and coming back and noise never recurred?? maybe the mechanic left a socket under a dust cover making all the rattling, and after it fell off; it's laying in your parking spot.. this is a hypothetical, but possible.. (and way better than internal engine damage)
if the auto is still operating mostly as it was prior to 'non-service'; you may have caught it to prevent further damage.. however, i would definitely start documenting everything with paperwork, pictures, video, etc. in case you have to file a claim with the Better Business Bureau (i think down under it's the ACCC)..
and document also pics of where you normally park; at home, and at work, to show no large puddles of oil as far as a major leak; and video tailpipe at start-up, and after regular commute for a few seconds to show smoke/or lack of..
all of this still doesn't help prior to, just going forward.. if the shop did not perform work that you paid for, it is hard to know how to prove; UNLESS, you have another shop to possibly go to to have an inspection of items in question:
for instance, if your receipt shows drive belt change- most all manufacturers place date codes stamped on belts near part#, brand, etc..
example: if your belt still shows a mazda logo and a manufacture of 3months prior date code of your vehicle, this belt went on in factory.. or if you have an old receipt showing a prior old service showing a Wix or Fram oil filter used, and the shop stipulated in papers they installed a Mahle filter; this would show neglect, and lack of service performed.. it may be that even if they honestly did the work, that the oil pump might be going bad, or something else internally..

i hope this helps; however, it still does not repair the car, and find if any damage has been done; so it behooves you to, decide if you allow your shop to continue work, or find another.. "most" reputable techs and shops in america will even box old components for showing customer it was removed and replaced, and even on common items; such as oil filter, marker a month/year or miles swapped at on it.. to show themselves, or others, when it was last serviced..
hope the 3-speed is still 'zoom zoom'-ing after this ordeal..
please post further updates, as i don't want to say to change components that might have been serviced; if it is something else, and it is difficult to diagnose w/o more info..