That rumble you hear is trading volume, which is currently tracking at record levels exceeding Wednesday and Thursday's epic totals.
It's very difficult to estimate so early in the day, but for the NASDAQ, EOD suggests the high 3.x billion shares. NYSE is above yesterday's all-time record of 2.8 billion.
Should volume back off from these quick-takes by the close, that would be interesting behavior as well. VIX almost tagged 30 before easing a bit.
A summer to remember for sure.