Credibility has taken such a licking around here that today's cheering news from Three Mile Island produced more of a whisper of relief than a sigh.

The cheering news was that the crisis and the threat of a major disaster seemed to be waning at the Metropolitan Edison Co.'s crippled nuclear plant three miles from here.

But the news was not cheering enough, apparently, to stop an "extraordinary" run on banks in Middletown and elsewhere in the five-county area, where withdrawals were reported heavy.

State banking secretary Ben McEnteer in Harrisburg, 15 miles north of here, said the "extraordinary withdrawals" - he gave no dollar amount - were made by people evacuating and by others who feared a major disaster.

McEnteer said significant numbers of depositors were emptying accounts and deposit boxes, in the apparent but mistaken belief that their bank records would be lost if a catastrophe resulted from the Three Mile Island situation.

The run on the banks seemed to typify a reality: people here in Dauphin and surrounding counties have heard such a welter of confusing and contradictory reports since the accident last Wednesday that they will believe the emergency is over when they see it is over.

The number one question on most minds related not to the possibility of massive radiation leaks from the plant, but rather whether Gov. Richard Thornburgh would order mass evacuation of this area. As of this evening no order had been given.

Irv Strohecker, the city's tall, strapping recreation director, said, "President Carter's visit Sunday eased some minds, but people here want to know if they will have to leave and, if so, when that will be."

And that, of course, was the question that no one in authority - credibility problems or not - could answer until a formal end is declared to the crisis at the nuclear plant.

Strohecker's wife, like many other young mothers, took their children out of the area last week when Thornburgh advised pregnant women and preschool youngsters to leave as a precautionary step.

Mrs. Strohecker drove back from Shamokin, 70 miles from here, this morning to pick up a supply of clothing. She said hello to her husband and left town again.

Donald (Butch) Ryan, Middletown's civil defense director, took a break from another of his 20-hour workdays and summed up what he had been hearing.

"We put our faith and confidence in these officials," Ryan said. "They say it's stable at Three Mile Island, but that's not enough for most people. They want to know more."

With many of the town's residents gone, Mayor Robert Reid continued his nightly curfew and reiterated his order to police to shoot looters.

George D. Hinkle, operator of a sporting goods store, took his own precautions. This evening he removed firearms and other valuable merchandise from his shop to a safe place.

William Creason, a city patrolman who said he had had only eight hours sleep since Wednesday, downplayed the idea of trouble after dark. "The only problems we've had were fights in bars. But we have that when there's no emergency," he said.

Creason's family was another that decided safety meant being somewhere else. When he got home yesterday he found a note saying his mother, sister and grandmother had gone to stay with a relative in Oxon Hill, Md.

The intense interest in reliable information drew a number of Middletown residents to the borough hall this morning to hear Harold Denton of the Nuclear Regulatory Commission brief the press on developments.

What they heard was that the situation at Three Mile Island had improved markedly since Sunday. What they saw was something else again - the jabbering and jostling of a huge squad of reporters and TV cameramen drawn here by "the event," as Denton has come to call it in the bureaucratese of the NRC.

The statistics and the raw data are there, but making head or tail of it is as difficult for Middletownians as it is for the news gatherers, some of whom are experts, most of whom are generalists bopping along after a good story.

One of the most helpful events since last Wednesday was a little act of public service offered by state Rep. Stephen R. Reed. His aides were papering Middletown with leaflets that provided a glossary of nuclear terms and a down-to-earth account of what went wrong at Three Mile Island and why it is dangerous.

The confusion felt in Middletown and the crush of roving bands of TV cameramen was repeated at the American Red Cross emergency shelter in Hershey, about 10 miles from here.

Some 140 men, women and children are being housed in a large sports arena, enveloped in the sweet aroma of molten chocolate, wondering when they will get to go home.

As a matter of fact, some were not all that keen on going home. They like it in the arena where they are fed well, showered with goodies and snacks, and entertained.

"All five of my children like it here," said Sheila Stevens of Harrisburg who went to the arena Saturday. "Free ice cream, magic shows, color television, three hot meals - it's excellent treatment."

"But," she added, "I'm still puzzled. All I hear about is a bubble. I'm so confused I'm looking for the right answer. Is this going to hurt our milk?"

The bubble she alluded to was the hydrogen gas bubble giving technicians a fit at Three Mile Island. The milk she worried about was the milk that radiation could contaminate.

Tony Maldonado, 14, one of 22 Maldonados whose cots line one wall of the hockey floor, was not much worried about anything. He offered an interviewer free ice cream and said he would not mind staying in the arena for a while.

"I'm not worried about this," he said, "but I'm scared for my mother. I don't want nothing to happen to my mother."

While concern about Three Mile Island is thick enough to cut with a knife, it's real impact may not show up until "the event" is concluded.

Bob David, a case worker with the Dauphin mental health program, said the crisis intervention center has had little extra business since Wednesday.

"People are busy, they suppress their worries, they have no time to think. When this dies down, I think it will hit us," he said. "A lot of people will want to talk about what they went through, that they felt inadequate in the crisis and embarrassment at being scared."