That KNOCKING sound...

Ok so I have a 2010 Nissan Pathfinder that,
before it was mine, it was unfortunately my
husband's (originally, it was my husband's
mother's, she babied it and took it to the
dealer for any and everything religiously
before giving it to the niece) 18 year old
nieces, whom had a horrible led foot, and
never, not once, even opened the hood. And
when she got it, it had around 100,000 miles
on it, when I got it after it was taken back
from her for being an idiot child, it now has
209,589. She still runs strong indeed but
she's got some issues and I'd be lying if I
said that I wasn't afraid deep down that it
was just gonna blow up one day.... Ok so
anyways, I've googled and googled and
googled some more but I haven't found
anything that sounds like what I'm hearing. It
comes from the front end under the driver
side area and it only sounds when I'm  
CRUISING between 6mph-18mph, never any
faster and the noise stops immediately
when I step on the brakes or the gas pedal,
and only when I'm cruising. And the noise is
a KNOCKING sound, KNOCKING, not
whirring, banging, clinking, clunking,
screeching, scraping, whining, hissing, JUST
STRAIGHT UP KNOCKING, like on a door.... I
had a friend(legit mechanic) look at it and he
said it sounded like an engine misfire, and
I've looked at multiple videos of my same
make model year of what an engine misfire
sounds like and it don't sound like my
knocking noise.. I have a video of the sound,
nota great one but you can clearly hear the
sound if your clueless as to what it could be
ask for the video and I'll send it to you.
please PLEASE PLEASE HELP ME!!!! Thank
you!!