Loud thud heard through speakers

Noticed a weird sound while listening to music today. A loud thud through the speakers during loud vocal passages. I'm not sure what's causing it but happened twice and on different records. One was Muddy Waters Folk Singer and the other Dire straits Brother in arms. 

Amplifier: Graaf GM50BSpeakers: Wilson Benesch ACTSubwoofer: Sumiko S9
At this time I'm not sure of it's the Sub or the Speakers. Did anyone experience anything like this before?

I noticed yesterday a thud through the speakers when I turned the electric oven off in the kitchen, which is around 8m from the turntable. No record was playing at the time but the MC phonostage was selected and I’d left the volume turned quite high. I used to get a similar noise in my last place when the fridge compressor started up. I think it stopped when I had a dedicated spur fitted but that coincided with moving the system 4m further away. I also used to pick up some radio broadcast in Russian at a low level but that stopped when I replaced my Target rack with something less metallic.
@yeti42  thank you for sharing your experience. How loud was the thud you heard? I was listening to music at moderate level, listening to Muddy Waters Folk Singer. When his vocals got loud is when I heard the thud and both times the noise coincided with Muddy's vocals getting loud.

My stereo is about 18ft from any kitchen appliance. The amplifier is currently plugged into a power conditioner too.
Ok, narrowed it down to the Sub. The noise is not coming from the speakers but from the Sub. Not sure why the Sub is popping so loud during loud musical passages even though there's hardly any low frequency information at that moment.
You need to better isolate your turntable. You're almost certainly getting bass rumble or feedback. If you post a modified version of this question over in the analog area you'll get a ton of information about how to overcome the issue. 
@cal3713 Thank you for the suggestion. I'll try playing same tracks from a digital source today and see if I can induce the pop. If there's no pop then as you said it could be a problem in my analog chain.
Update. It was bad buffer tube in the phono stage. Replaced the tube and all is well now.