Usually that's caused by laminations in the transformer vibrating in sympathy with the surge in current through the transformer as you turn on the amplifier. Normally it is not a real concern unless that sort of thing bothers you. I have a Mark II that has been doing that since the mid-1980s when I first rescued it from a junkpile and modified/upgraded it. That amp is still in my main system today and is playing music right now.
However your amp ISN'T working any more, so further investigation is warranted. Check the fuse first. I forget if the ST-70 calls for a slow-blow type; if it does, and you put in a standard-blow type, it may have blown on that initial current surge. If a new fuse doesn't restore the amp to life: with the amp unplugged, check the primary and secondary windings of the transformer for continuity. You may find that one of them has opened up, which would be bad news. If you feel up to it, unplug the rectifier tube, plug in the amp to AC, and switch it on; CAREFULLY (high voltage on the B+ winding!!!) check the secondaries for AC voltage. Let us know what you find.
I'm a bit concerned that your amplifier did NOT make the SPROING noise previously.