If my exposure time is 30 sec & if I set interval time 31 sec or 32 sec, then it is not working properly. Camera should take each shot 1 sec or 2 sec gap, but camera takes long time between each shot. But strangely if I set interval time 33 sec, then it's working properly. Camera in manual mode, long exposure + high iso noise reduction turned off. Is it normal ?
That is very normal (and correct for any camera), because the 30 second exposure is actually 32 seconds, and you have to set the interval timing slightly longer (33 seconds). Time the 30 second exposure yourself, and you will see it is actually 32 seconds. Which is exactly correct.
Because, stops of exposure are each 2x longer, so for the light meter to compute correctly, the actual precise goal of shutter speeds must necessarily be 1, 2, 4, 8, 16, 32 seconds. The marked numbers are just nominal values (not necessarily actually existing as such), just easier for human brains. :) The timer interval must be set longer than the shutter speed used.
The same 2x sequence concept is also true of fstops and ISO and fractional shutter speeds, 1, 1//2, 1/4, 1/8, 1/16, 1/32, 1/64, 1/128, 1/256, 1/512, 1/1024 seconds, etc. That's just how cameras have to work (binary steps). And also we wouldn't like to see a third stop slower than 1/500 second marked as 1/406.4 seconds (but that is what it necessarily is).
My site has charts of these actual values.