IMHO the noise is nothing to do with oil viscosity, it may be knocking or pinging (pre-ignition )- are your plugs in good shape (especially the rear 4 which never hardly get changed) if you don't know when they were changed it is probably a good idea to replace them, rear ignition coils may fail as well, but they normally cause uneven running, standard spark plugs are not expensive (plenty of information on this forum about changing rear plugs) - maybe try a tank of high octane fuel which, but if it is rear plugs to blame it may not help.
I personally would not go lower than 5W-30, I do not trust the 0W-20 and lower oils now being peddled as fuel saving oils, car makers will use anything in the laboratory to get the last 0.1% mpg gain, with scant regard for future engine wear. If I lived in Siberia or Greenland I may use thinner oil, but in UK - Naaaahhh.
Makers used to sell (maybe still do) sell 'high mileage oil' (no it does not do a higher mileage before it needs changing, it is higher viscosity) for use in older engines which have bigger bearing clearances due to wear - the thicker oil maintains oil pressure (where the thinner oil just passes through the gap like water). If your engine has done a few miles I would not change the oil as long as it is a good quality one,
maybe this is why your mechanic used thicker oil - why not ask him ? if it is a semi-synthetic you should not have any problems starting the engine (thicker oil does produce more drag on engine when starting) - you may get a slight reduction in mpg, so depends on how many miles you normally drive if that is a problem or not.
https://www.cenex.com/about/cenex-information/cenexperts-blog-page/oil-and-lubricants/high-mileage-oils