99 crown Vic sounds like foghorn ?

Would a clogged trans filter cause this or is this a deeper problem ? Pretty
sure it's transmission related ; runs , drives and shifts fine . So recently my
transmission started making this sound , deep horn type sound ; every once
in a while at 1st , but pretty regularly now . Got a smoking deal on this car
when I worked at a dealer ,$1100 ; so I'm reluctant to spend a lot of cash to
fix it . Now every once in a while it will stall when I come to an abrupt stop . I
have put only 30,000 miles on it in 7 years , had 134,000 on it when I got it .
Been a great car , just can't see spending a $1000 to fix a $1000 car . Thanks