thanks for all the comments..
found out that the smell comes from the filter when it gets moisture in it, we've had very high humidity here lately..
part of the filter is made up of small beads and its the beads that smell when they get wet..
the filter was removed and the rubbery stale odour was very noticeable..
the filter was repacked with new materials, the humidity is back down to normal, so the compressor is back in business, so to speak..
maybe the beads release the smell to indicate that moisture is in the filter, like the rotten egg smell in natural gas, used as a warning that something is not right ???
my tanks have been emptied, inside cleaned to remove residual smell, will put some air back in them today and hopefully go diving tomorrow..
its been an interesting learning experience for me..
and an lazer beam thermometer/heat gauge will be purchased to monitor the working temp of the compressor..