Air Conditioner Causing Thump

When I turn on my A/C, after only a few seconds, a loud thump seems to comes from under the floor–maybe the firewall. You can feel it! It only happens when the truck has been sitting enough to return to ambient temperature. I had a person watch the firewall and under the truck, but nothing is obvious. Any ideas?
I’ve looked underneath thinking I might have a Boy Scout or little old lady who were too slow in the crosswalk, but I can find nothing. (Please, it’s just a joke)
There is a second issue: When I’m driving with the a/c on, when climbing hills or an otherwise load on the engine, there is a high-pitched whine which I’m going to deduce to be the bearings on the A/c pump. Belt is only about two years old, is clean and tight. I only drive about 2500 miles per year. No, not a typo, I seldom drive.

the thump you here is the blend door closing.

Thank you! That was a quick response. Is this of any concern, or just an anomaly of the truck?

its normal. the door closes to direct the cold air out your vents

Thanks. It just seems excessive in power. But that’s one thing I don’t have to look forward to a problem with.

as far as your whine. when you changed the belt did you also change the belt tensioner and idler pulley? if not they may need to be changed. when you put the a/c on it may be putting just enough tension for it to make it whine if they are just starting to go. just a thought.

Actually, the belt tensioner is why I had to change the belt. The bearings on it froze and the friction melted the plastic wheel. The belt really did not look at all damaged at the time, but I’m always prudent in such situations to use caution and get out front of any potential fallout. Thus the new belt.

 I have a stethoscope and will try that next. it's not urgent. I drive so little, I can use the A/C sparingly. Summer will also be winding down soon.

It’s possible the hvac door is sticking a little too. That conceivably could make it close with a louder than expected thump. OP, I’m presuming you’ve owned this vehicle for some time. Have you noticed the sound, or the loudness of this sound has changed of late?

I’ve had it about 6 years and the noise has increased over a period of time. It’s actually a physical thump that carries a small vibration. I’ll google the schematics and look into it. Thanks!

If it isn’t the door making the noise, another possibility is the AC compressor. It looks like it is spinning all the time, but sometimes it is just freewheeling, and other times it is engaged and turning the compressor. When it switches from one mode to another, that could cause a thumping sound.

Thanks. I did have someone listen under the hood when I turned it on and it appears to be coming from the firewall.

My Taurus had a similar issue. The little servo motor that moved the blend door had plastic gears and it broke a tooth or two. It would thump once or twice every time it actuated. You could google Blend Door Actuator for your car or search YouTube to see where it is located and just temporarily disconnect it to see if that’s the issue.

That’s a good possibility. The “thump” is exactly what I would expect in this case–echoing through the dash and plastic tunnels. Also, checked YouTube and found further evidence of this.