Random Crackling after mp3 mixdown bounce

Hi all this is my first post in a forum and would love if anyone could help with this. I did do a search and found similar issues but none really hit the spot but apologies if this has been raised before.

I am using Cubase 9 Pro on PC Windows and just finished mixing an original track, exported as wav and reimported as a new session to add some mastering effects.

When I playback either the mastered track or the original session in the DAW I don’t hear any clicks or cracks, however i then mixdown to mp3 and send to my email to listen on different headphones/car stereo etc

Here’s the weird bit - only sometimes when I play the track back I get a cracking noise at the same part of the song every time - if I rewind and replay it isn’t there. If I close and go back into the track it isn’t there. I’ll play it again a few hours later and there it is again? I have tried reopening the session and tweaking a couple of bits but the same result every time at the same point in the song. Finding it bizarre to say the least how on the same mp3 it sometimes cracks and sometimes doesn’t - any advice greatly appreciated.