Dave Grohl's ex Kari Wuhrer says she isn't surprised to learn about his lovechild ... and says there's been chatter in Hollywood about his antics for years.
The actress tells TMZ ... she doesn't know the identity of the woman who Dave says he recently welcomed a baby daughter with -- but says there have been "rumblings" around town about the rock legend being unfaithful to Jordyn Blum, his wife of 21 years.
Kari and Dave dated in the late '90s ... with KW telling us the Foo Fighters frontman pursued a relationship with her while he was still with then-girlfriend Tina Basich.
Kari says she learned the truth of Dave's two-timing ways when she called in to "The Howard Stern Show" and vented about her relationship with the radio DJ after Dave had yelled at her. Tina caught wind of the interview, which prompted a breakup between the snowboarder and the Nirvana alum.
George Clooney's ex-fling added ... "I was letting the cat out of the bag ... I never heard about her. He never spoke of it."
The drama ended things for Kari and Dave, as well ... with the actress telling us she never heard from DG after the yelling incident and the 'Howard Stern' situation.
Though, they did run into each other years later in Los Angeles ... as their respective kids went to the same play place.
She continued ... "He would just look at me like I didn't exist. I just felt like I knew he was scumbag after that."
Kari's now advising Grohl to apologize to his exes in the wake of his baby bombshell ... and she hopes he steps up for his new daughter -- which he indicated he'll do in his statement on the matter.
We've reached out to Dave's reps for comment ... so far, no word back.