The wail of diesel truck engines is beautiful music--especially to Charles County's economic strategists.

As more and more businesses discover Rte. 301, the artery to southern Maryland that connects the Baltimore-Washington market with Richmond's, county planners and developers see signs that Charles County's own market is steadily expanding.

Planners have been priming the county as it emerges from three centuries of agrarian economy, creating industrial parks and wooing retail establishments, even competing with more developed jurisdictions in the quest to lure computer-based industry to the area.

"We're letting them know that there is some place somewhere called Charles County," said Raymond T. Tilthman, executive director of the county's Economic Development Commission.

Rte. 301 once lived the high life as a booming strip of restaurants and gambling clubs, when it was a main corridor for north-south traffic. But in the 1960s laws were passed that took away the slot machines and brought Interstate highways that took traffic elsewhere. County officials say that they are working for the old highway's rebirth.

County commissioners this week brought two applicants closer to$3 million in industrial revenue bonds by holding a public hearing and, upon finding no opposition, closing it three minutes later.

The two projects--a $2 million, 92-room expansion for the Waldorf Holiday Inn hotel and conference center, and a $1 million construction of a tractor sales and service facility--represent the commercial aspirations of county planners.

Alban Tractor Inc., a distributor of Caterpillar equipment, would be the first resident of the 136-acre La Plata Commerce Center, one of several private business parks the county has begun promoting. Officials expect the facility to employ as many as to 100 workers.

Robert Blundell, the county Industrial Development Representative, said that the Holiday Inn "has excelled all expectations" and has had occupancies as high as 95 percent as corporate representatives base themselves there while on business in southern Maryland.

But problems remain. Traffic backs up around Waldorf, aggravated by ongoing road expansion designed to relieve it. Officials expect that conditions will improve in coming years with the construction of a bypass around the intersection of Rtes. 301 and 5.

The most expensive problem involves extending connections from the Matawoman waste treatment plant, which is operating at 10 percent capacity, to industrial sites.

"Water and sewer is not an easy problem," Tilthman said, adding that the county must press for federal grants to make connections that will attract industry and jobs.

For the Alpan Tractor site, Blundell said, builders will install a special solar septic system that condenses and dries waste. It will be, Blundell said: "The first use of the system in the state."

Meanwhile, Charles County officials see plans for a K-Mart, fast food restaurants and a Giant Food Store on Rte. 301 as the promise of still more to come.