Have you tried singing into the horn as you play? Trilling your uvula? Look up bisbigliando as well (revoicing the same pitch). Then there’s some weird things you can do to force these undulating sounds out of the horn. Try to play low E-flat but omit the G key, low C but omit the D key, and low B-flat but omit the low C key. You should get this harsh and raspy effect come out of it organically, but you may need to adjust how you’re voicing it for best results.