Mobil 1 is crap for an engine with this mileage, use SILKOLENE PRO S from opie oils on here.
Just me personally but I would not even put Mobil 1 in my lawnmower.
If recently rebuilt may have used cheapo tappets, and not INA which are oe kit, and still only around the £80 mark.
I would firstly change the oil for above and watch the noise disappear, it will be perfectly safe to use until you get around to doing it as well, the hotter the better.
tabetha
ps a warped head cannot cause the noise.