just flushed the coolant system using Shout Ultra Gel followed by Mercedes
Citric Flush. this is on a 1984 300DT. used a prestone back flush kit with the garden hose to flush in between Shout and Citric flush. A LOT OF BROWN CRUD CAME OUT!! filled the cooling system with tap water and parked it.

anyway, there is a tap tap tap sound coming from the front of the engine
(on the air cleaner side). so, removed the air cleaner but the engine
still makes a tap tap tap, more noticeable at idle. I've got a new radiator
but before I install it with new antifreeze I want to drive the car to a
mechanic that is about 3/4 mile from where I live so he can listen to
the motor.

question: will it hurt the car to drive it that far without the air cleaner and with only tap water in the cooling system?

If it is dusty out, I would avoid it. Also avoid lawn clippings and such. If you can drive it on a wet day, much less dirt will be suspended in the air.

As for pure water, as long as a drain and refill is part of the plan, I wouldn't worry about it. Time is as much of an enemy here as mileage.

There is a yes and no to driving without the Air Filter.

In general it is a bad idea incase something unlucky happens to get sucked in.

Otherwise a 3/4 mile drive should not cause any issues. So it is a judgement call for you to make.

I know I have often driven down streets where construction is going on and there is considerable dust and I have been glad I have an Air Filter.
Story Time:
When I was in Trade School I was sitting in a grassy are a crossed from the Diesel Shop Eating my Lunch. I was observing 2 Students running a 4-71 Detroit Diesel with out an air filter (it did have a about 1/4" mesh screen covering the Blower opening).
The Students left for Lunch and left the Engine Running (unattended running Engine is a bad Idea).
It was a gusty windy day and getting blown is a Full page of News Paper.
Most of the page got sucked into the Engine. Amazingly the Engine rpm dropped and suddenly picked up and the burnt bits of News Paper belched out of the Exhaust Pipe.
I ran over and found bits of charred News Paper.
What had happened is that the Mesh Screen had saved it. When the News Paper pulled through the Screen it was shredded into fine pieces small enough it did not do any obvious damage as it passed through the Engine.

I shut the Engne off and went to my class. I did not say anything and let the 2 Students worry about why the Engine apprently shut off by itself.

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84 300D, 82 Volvo 244Gl Diesel

3/4 mile...no worries.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

Get some antifreeze back into it, it's a corrosion inhibitor and helps lube the water pump seal and bearing

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I choose to be happy, even when the Mercedes is being awkward, lol