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James, Ford Technician
Category: Ford
Satisfied Customers: 3628
Experience:  Ford Senior Master Tech, Ford gas and diesel expert, ASE Certified
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I have a 99 f350 and its got this annoying clunking noise

Customer Question

I have a 99 f350 and its got this annoying clunking noise that sound like a damper door of perhaps something withthe bracks? The reason I suspect the bracks is that when i brack lightly, the sound goes away? Any ideas?
JA: Where exactly do you think the noise is coming from? And how long has this been going on?
Customer: It sounds like its coming from eoither the vacuum reservoir of perhapsin the ducting from the HVAC. Its a dull thumping noise that also gets worse during the rain -- smost times when its dry, I don't even hear it?
JA: Are you fixing your F350 yourself? What have you tried so far?
Customer: What I've tried is to separate the pasenger side wheel well fiberglass? inner fenderwell from the frame as I thought I was recreating the noise by "droping" that inned fenderwell onto the fram -- the vacuum reservoie is also bold to that piece? And yes, I am doing my own repairs
JA: Anything else you want the mechanic to know before I connect you?
Customer: No that's it
Submitted: 9 months ago.
Category: Ford
Expert:  James replied 9 months ago.

Hello, does the thumping noise happen all of the time or just when going over bumps in the road?

Customer: replied 9 months ago.
All the time when its raining and wet. when dry out, I don't here it--
Customer: replied 9 months ago.
I have had the brakes and suspension looked at by professionals. The noise isn't in the either according to them. It does sound like the inner fenderwell passenger side is getting "bumped" by something? or its in the ducting of the HVAC? The nosie is up high in the engine compartment, behind the passenger side wheel.
Expert:  James replied 9 months ago.

If the sound is coming from the passenger side higher up I would inspect the vacuum pump to make sure the bolts haven't vibrated loose causing it to get knocked around. I would also inspect the vacuum canister and it bolts to the inner fender and can tear itself out causing issues. Once thing to look at are the sway bar links just to be on the safe side. A lot of people over look sway bar links due to not bein sure to tell how they are bad. When you grab onto the sway bar link you shouldn't be able to wiggle at all by hand. If you can they need to be replaced and can cause similar noises to what you are hearing. I would wiggle everything mounted on the passenger side fender and see f anything wiggles. If anything doesn't seem solid It can cause your noise. Once you make sure everything is secure your noise should go away.

Customer: replied 9 months ago.
The sway bar links were just replaced at 150000 and I only have 1000 miles on after that. I think you may be right in the vacuum pump motor or perhaps a vacuum dashpot on the HVAC under the hood may be loose. Is there a vacuum dashpot for the HVAC under the hood behind the vacuum reservoir?
Customer: replied 9 months ago.
Is it possible for you to email me with an accurate vacuum schematic for a 1999 F350 with the 7.3 Navistar engine? I think that would really help me, because when I hit the brakes and pull vacuum, the clunking definitely goes away. ThanksRoger
Expert:  James replied 9 months ago.

This vehicle should have hydroboost braking and not a vacuum diaphragm. The vacuum pump controls automatic 4wd is equipped. The other thing that vacuum pump operates is the vacuum solenoids for the blend door, and recirc door on the inside of the vehicle.

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