After about an hour the tv soaun makes a

The main board has problem, or the main power supply has bad electrolytic capacitors on it.

Check the speaker/s. If found OK; check and replace the damaged component/s at its audio output section circuit board. If needy, replace this board as card basis. If you wish to get some details; check the site linked here. Surf the site with patience. Pull up older posts. Surf the site with patience.
http://electronicshelponline.blogspot.com/

provably your power supply. You can check for bulged capacitors or a part of your board that looks discolored.

You can get get parts for this tv at www.interliquidparts.com

put a towel underneath. that will disipate the sound of the cabinet poping. not overheating normal for the cabinet when cooling down to shrink and make poping noise.

If the sound FLUCTUATES up & down, try turning off the AutoVolume in the sound menu. You didn't say if it's one or all speakers crackling. Crackling is either a speaker or circuitry issue. Adjust the balance away from the speaker that crackles, if it stops, that speaker is probably "fried". If it still crackles, I'd turn off the speakers and run the sound thru a stereo for the quick, cheap fix. My Samsung (10+ years old to be fair) crackles due to a circuit issue but if I SMACK the left side speaker (behind which is the sound board and side panel connectors) it stops for a while. Maddening. I think I broke it squeezing it into the furniture it's in, pressing on the protruding RCA plugs. Dumb design.

It is the audio board inside

youre having deflection problems could be related to the flyback. But this type of repair is better off with an experienced tech that has the proper equipment and knowledge to diagnose this chassis

From the speakers or from the inside of the unit. On all modes, eg. normal tv + digibox/scart input???

I just bought a used 60 inch rear projection tv. The picture is fine except I have a crackling sound coming from the front of the tv also. What's the deal?

On some of these Hitachis they have a surround sound option, there is an additional board in there that goes bad and needs to be repaired. suggest a repairman or service center.