Clicking / Snapping Sound from Dashboard

This is about my daily driver. The GTA is in the garage in all this snow lately.
I have an 85 Chevy Caprice with 343,000 miles on it ( yes 343,000) . Suddenly,
I am hearing a clicking / snapping sound from the center / right of the dash during acceleration, and also sometimes during deceleration, and also when in Park, at about 1500 rpm. The noise comes and goes, but usually it is there during high end of the gear during accleration. Sometimes not there at all, even while accelerarting. It's an unevenly spaced click / snap sound, like a spark knock.
At first, I thought my spark knock was coming back, but a spark knock doesn't occur in neutral, and during deceleration. So I am thinking it might be : u-joint... transmission... distributor... exhaust... or speedo (but would speedo click in Park?)
It happens with and without the heater fan on.
I can't see anything unusual under the car or in the engine compartment. Have not been able to drive it up on ramps yet to get a good look.
It started 2 days ago, out of the blue, and is about the same as it was 2 days ago.
Sounds like it is coming from the dash, but I guess it could be from the trans hump too.
I hope it's not the trans.
ANy ideas welcomed ! Thanks
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Lecher9000 Spilled my beer when they jumped on the table and

Is it possible to have someone drive it the same way you do, while you sit in the passenger seat?
This way you can narrow down where the noise is coming from...
NOI
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Yes, I could. Good idea. But since it makes the noise at idle, I just leaned over and put my hand on the cover for the heater core ( I have my glove box out and can reach down in there), and I felt the vibration there while the noise was happenning, so I am hoping it is just air bubbles in the heater core. I had a leak at the heater hose / heater core connection yesterday. I tightened it up today, but the noise is still there. Maybe it takes a few days for the air bubbles to work their way out. Just a guess though.
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Might sound wacky, but check to make sure your flex-plate to TC bolts are tight and all there...
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Would I have to drop the pan to get to these bolts ? Or are they accessible from the outside ? Thanks.
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There's an access cover between the oil pan and the transmission pan. Should be easy to get to...depending on how the exhaust is routed.
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Suddenly,
dash
in
all,
a
doesn't
Have
days
trans
Do you park where leaves can get into the fresh air intake for the heater and a/c ? I think your fan always runs on low even when the heat or air is turned off so mabey you have leaves rubbing the fan in the heater core/air box. I have a '93 GMC truck and I have this problem. Hope this helps.
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Yes, I have leaves aplenty. But I am 99 percent sure the fan is off when the heat is off.
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Lecher9000 Spilled my beer when they jumped on the table and

OK. I went back to look. You have an 85 Impala/Caprice. Is this the boxy looking one? If so, the only way to turn the fan off was to flip the direction selector to off... One way to make 100% sure is to pull the fuse for the fan temporarily while it's acting up, to see if the problem goes away. (If it does, then it's the leaf issue. If not...good luck.
and for the record, I have had this happen before. For some reason, I always forget it... <G>
NOI
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