308 dipstick rattle

Is the mount screw/bolt missing?

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Rattling / vibration noise on passenger side could be several items. You are very sure it is the dipstick?
Have you removed it, drove it to 4k and see if the noise is still there?
Reviewing my past service receipts, I found one for rattling on passenger side which came from a cracked exhaust sampling tube.
The usual Loud vibration sound near 4k rpm on the passenger side is AC hoses resting on the body.

 

If it is a US car, it may be the packing around the exhaust headers.

 

More likely, the AC hoses vibrating against the body air intake when you get to ~3500 rpm, then stops above 4500 or does it continue above 4500?

 

The AC lines really should have a (now disintegrated) foam pad against the body panels, I bet that's going to be it.

The front sides exhaust sampling risers were also mild steel and been lots of threads with them breaking and giving up.
So another but less likely suggestion, IMO they could break, but I don't see them rattling against anything but the engine you'd never hear it in the cabin.

 

Make sure all your inner fender fasteners are present and well landed too.
They strip out or go missing as time wears on....

Oh one final thought, IF the engine mounts have sagged to the bottom, the RH header can actually drop for enough to hit the frame, so check there's clearance and as posted above the aluminum wrap on the headers can loosen as the asbestos packing dusts out.....

 

To stop the rattling of the AC hoses on the body near the air intake, I purchased the foam used to insulate pipes from the local hardware store and used tyraps / cable ties to keep the foam in place. You need to remove the fiberglass rear passenger side wheel well to get to it. My hoses were laying right on the intake portion of the body. After I installed the foam, no more rattling sound.
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I'm a huge user of Armaflex, as I get scraps (even huge rolls!) from work.

Unlike the ancient open cell foam of the 70s, it's skinned, closed cell, sheds water...….so a scrap of that tie wrapped around those flex refrigerant lines in the quarter panel, may just well "silence" your pest!!

Lets just say when my tools hit the drawers, they say "ahhhhh" and make no clank!!

 

Yes, those lines to the front snake down the rocker panel (with great difficulty) and then bend double bends straight up to come popping into the engine bay!!

His pipe insulation is basically what I am saying but wrapping sheet around the air intake snorkel would do the same thing.
The snorkel and fender scoop gives the rattle the sonic amplification it needs to be loud.

I'm an expert, on :loud".....

 

They will have the residential pipe insulation.

Armaflex is used on HVAC equipment so it does not sweat the moisture from atmosphere, so the sheets will be at Johnstone Supply or maybe Grainger.
I hate that you can come by my "Scrap pile" as I keep it from 3/8" up to 2" and they actually go thicker!!

Since we chatting the OTHER place you need it is the OEM seals of the radiator, there's a bottom piece, two side seals and even two "little blocks" on the radiator corners. Once those are gone or knocked loose the air is leaking around rather than thru both coolant and AC coils, to ill effect, sealing it helps with several known issues, both crappy AC and overheating in summer.

The two strips on the hood are also best replaced, my hood "pops up" 1/4" when released and it's tight to the top of the radiator.
I need to exercise her I will get some pictures.

 

We buried two city blocks of chilled water lines at a hospital, and used enough 1.5" to wrap a building, then covered it up in dirt!

That was my first time to get the "light bulb" on....

 

My photo was before installing my foam around the parts of the AC hose touching the intake body on the passenger side. In the past someone installed foam which is visible in the background. I removed the old foam since they guessed wrong on where to install it. Pretty sure it is AC hoses rubbing on body at intake. Resonance of 308 engine is 3500 - 4500 rpm for vibration.
Keep us posted.

 

Follow the hoses from the AC compressor as they go toward the right side. It will be easier to see if you remove the black fiberglass intake tube. You will see a hose or both hoses resting on the air intake tubing or air intake part of the body. Where you see the hoses touching or possibly touching the intake is where the foam insulation needs to be installed.
It will be behind the area being pointed to by the red arrow. I will PM my phone number, also.
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