Troublesome

I have a 97 civic dx a problem, when I start the car when it's cold I get a lot of rattling in the engine but after a couple of minutes it's stops until I takes off then it has a tapping sound until it gets up to 30mph & once the engine gets to normal temp it stops & I don't hear it anymore, unless it sit still all day. Have taken the car to many shops but no one can tell me what it is, have had valve adjustment, timing belt changed, tune-up but it's still there.

RaceRx7

Is it a subdued moderate/rapid clank, clank, clank, or a very quick sounding rattle, rattle, rattle (almost like the sound of marbles being rattled in a can filled with water)?...

I'm betting it's a clank noise, right?...
Now, you have to determine if the noise is coming from the bottom of the engine or the top!...

If it's coming from the top of the engine, then it's most likely "piston slap"!... But, this should've been diagnosed when your valves were adjusted!... Your mechanic SHOULD HAVE performed a compression test!... If he/she DID NOT,... then, it's time to find another mechanic -- a COMPETENT one!...
Metal expands when it's heated,... so your pistons, piston rings, and cylinder walls expand, ever-so slightly, to reduce the gap between your engine's cylinder walls and the pistons/piston rings!... Hence, you only hear the noise when your engine is cold!

Do you also hear the noise when your engine is "under load", as in when you're stomping on the accelerator pedal attempting to pass someone, or when you're going uphill?...
If you do,... then you have severe engine wear!...

How many miles are on your car?

If the noise seems to be coming from the bottom of the engine...
then, you probably have worn "crank"/connecting rod bearings, or worst case scenario, one or more "spun" crank/connecting rod bearing(s)!... That's NOT good!...

The reason you only hear the noise when your car's engine is cold, it's because the oil drains out of the engine and tends to settle into the oil pan, after your car has been sedentary for long periods of time!... Once the oil gets sucked back into your engine and lubricates all the engine "internals",... the noise is gone!...

A quick way to diagnose where the noise is originating, without tearing your engine apart,... let your car sit in the driveway overnight (or anywhere where you can quickly and easily jump out of your car). The following morning go out to your car and open the hood. Get in the driver seat, make sure that your car is in park/neutral with the parking brake set, keep your driver side door open (you'll have to quickly jump out of your car), start your engine, and quickly jump out of your car as soon as your car starts. Quickly go around to the front of the car and listen for the noise under the car and over the car. Keep moving your head back and forth from listening for the noise under the car, to listening for the noise from your head being placed over the engine. If it seems that the noise is louder when your head is placed closer to the ground, then your problem is most likely the crank/connecting rod bearing(s)!...

BTW,... is your engine oil level where it should be?...

Hope this helps!.