Put the attach hall-interrupt in the setup routine, not the loop.
and, measure the milliseconds T1 before the attachment
In the loop, display the RPM.
In the interrupt, measure the milliseconds T2
do not enable or disable the interrupt, just leave it going.
If T2 > T1 then Calculate RPM = 60000 / (T2 - T1)
then, set T1 = T2
to wait for the next interrupt