Tornadoes in Illinois and low thunder rumble nonstop for 30 minutes or more

We had a very wierd experience today as the storms went through. About 10 minutes before sirens for tornado warning went off we heard what aounded like thunder. However, the thunder sound was non-stop for more than 30 minutes. There was never a lull or break in the thunder aound. It sounded like thunder or a plane was overhead above the clouds makinf noise for over 30 minutes as the storm approached and went through. I don't know what it was. Never heard nonstop thunder ever. A long continuous rumble. Anyone know what it could be? Wasn't the wind, wsn't thunder following lightening. Almost sounded like a thunder machine was running nonstop.

Finally as the storm moved east, the thunder followed, still rumbling nonstop until we no longer heard it due to diatnce.

We live in Frankfort IL and the storms came theough about 1 pm and we had tornado warnings and later heard a tornado touched down nearby about a mile east and northmof us, but even that wasN't what we heard.

Anyone know what this might havw been?