NEW YORK, SEPT. 25 -- The settlement earlier this week of improper bookkeeping charges against a former aide to Ivan F. Boesky was part of a strategy by federal investigators to hide the direction of their wide-ranging probe of Drexel Burnham Lambert Inc. and its junk bond specialist Michael Milken, sources said today.

The former Boesky aide, Setrag Mooradian, settled charges with the Securities and Exchange Commission that he falsified Boesky's accounting records of an attempted 1984 takeover of Fischbach Corp. by financier Victor Posner.

The decision to have Mooradian settle charges related to Fischbach was closely coordinated between officials at the SEC and the Manhattan U.S. Attorney, sources said.

The Fischbach case was chosen from several possible charges because they do not reveal which Drexel transactions are under the closest scrutiny by the government, the sources said.

Boesky's role in an alleged Fischbach stock trading conspiracy with Milken was disclosed last April when Boesky pleaded guilty. Though Milken was not named in that cases sources said he was one of the co-conspirators referred to by the government.

Limiting Mooradian's settlement to the Fischbach case kept secret other transactions being investigated now, according to sources familiar with the investigation. Prosecutors believe the circumstances of the Fischbach deal reveal motivations for possible wrongdoing by Milken and Boesky.

Boesky has told the government that a $5.3 million payment he made to Drexel in March 1986 was part of a scheme to reconcile accounts in an illegal trading arrangement. Boesky has told government officials he agreed to purchase Fischbach shares after Milken promised to reimburse him for any losses he incurred.

Drexel has repeatedly said the $5.3 million payment was for legitimate investment banking services and it has no knowledge of wrongdoing by any employe. Drexel officials say the money was properly accounted for.

Mooradian agreed to cooperate with SEC investigators after being granted immunity from criminal prosecution but before agreeing to terms of his SEC settlement, sources said.

Mooradian has been barred from the securities business for one year. He has specifically agreed to testify in future civil or criminal litigation arising from Boesky's allegations about Milken and Drexel, the sources said.

The SEC's investigation of Drexel and Milken is not confined to allegations made by Boesky.

The SEC has recently inquired about Drexel's involvement in some deals that were contemplated but never made public in early 1987, after Boesky's plea bargain last November, in which Boesky was not involved, sources said.

SEC investigators have also continued to probe Drexel's role in the 1986 takeover events surrounding TRE Corp., which was eventually acquired by Aluminum Co. of America.

Earlier this month, SEC investigators questioned the former chairman of TRE by telephone for about two hours, sources said.

Meanwhile, Drexel officials find themselves under increasing legal and economic pressure because of the progress of the investigation and disclosures of some of its details.

Drexel officials have repeatedly denied any wrongdoing, but a senior executive recently said the firm might consider a civil settlement with the SEC if it could avoid criminal charges against the firm and Milken.