It doesn't cause smoke but it creates a very loud humming sound for 5 or so seconds before the whistling sound starts, then 2 seconds after that there is a huge explosion. (timings from counting so not all that accurate)
Because it piqued my interest, I loaded up my testing save game and I think your ISU may have been a lot more than just slightly damaged before. The explosion leaves just a little bit of health on a Stug (480 health) and about two thirds of a Panzer 4's health (640). A ISU has 960...
Still, there may be some modifiers for it. I also tested it vs. enemy buildings and looked up the stats in the files and on
http://coh2-stats.herokuapp.com/ and the 750 damage stated in those just doesn't seem to be true. The friendly fire multiplier also seems to be 1.
Any of the other number crunchers help me out with that?
Nevertheless, it is a very powerful ability and when hearing the noise moving all your units that your enemy may know about as fast as possible is important. Normally his target is pretty obvious.
If you played CoH1, think of it as CoH2's V1 rocket.