Mysterious Braking Boing?!?

hi volvo guys...i have a 1990 740 with a mysterious noise while
braking. it sort of sounds like a boing, others think it sounds like
a low note on an out of tune piano. it usually only happens when i am
braking from a slow speed (parking, stop signs, etc.) i can feel a
little difference in the brake, a little vibration, a little more
give. there's also i squeaking from the front left wheel while i
drive. my mechanic has replaced the brake pads, master cylinder,
bushings, and hub over a period of 3 months. after the last visit it
stopped for a month and then today came back. anyone have any idea
what it could be? is it the calipers? is there a piano in my braking
system? will i be crashing through stop lights? i'm also out of town
and away from my mechanic. find someone to work on it now or wait a
couple weeks until i'm back in town?

i'd appreciate any help!

 

Had similar problem a few years back, not on a Volvo but on a '93 Magna -
turned out to be the brake pedal return spring making the odd 'boing' when
the brake pedal was depressed. If braking hard say from 30+mph it didnt
happen - only when tootering along.

Went on for months driving me up the wall - went away on its own after a
while.

Cheers
Mal
Oz

 

I don't know if this applies to your 740, but many years ago, my 164 was
making very similar noises just before the driver's side lower ball
joint separated and dropped the suspension onto the pavement. It was
making a creaking noise when steering and the boing when stopping. The
creak was the dry ball joint and the boing was the spring shifting
slightly due to the looseness in the suspension. I know that your car
has a completely different suspension, but if it were me I would check
ALL the suspension components NOW. It was no fun jacking it up and
replacing the ball joint on the side of the road, and I'm just glad mine
let go pulling away from a stop and not on the interstate

 

thanks for the posts so far!

....speaking of messed up suspension...last fall my front drivers side
strut completely tore loose. it almost stabbed a hole through the
hood before i got it to the mechanic. it's been eight months since
then and the boing noise just began a month ago. but i wonder if that
could have messed some things up that are just now voicing
themselves. i'm just confused why it stopped for a month after i got
the hub and bushings replaced and now is suddenly back.