Response: If you're not in custody (i.e., if a reasonable person in your situation would believe he/she is free to leave without being detained by the agents), you're not entitled to Miranda, which means you don't have the rights to silence or counsel. However, you can always refuse to answer questions posed by law enforcement officers; that's an outgrowth of your Fifth Amendment rights. So you can simply tell the agents you don't want to talk to them, and walk away. At that point, they will either have to arrest you (which will require that they have probable cause, already, to believe you've committed a crime) or have a prosecutor subpoena you before a grand jury, where you will have to answer unless you can take the 5th as to each question (meaning, you'd have to show answering would implicate you in a crime).

Can such silence bring about obstruction of justice charges?

Response: Silence will not, but if you lie, you can be charged with obstructing justice or lying to an officer (depending on the jurisdiction you're in). And a number of courts have held that pre-arrest silence can be introduced into evidence at trial, as the basis from which the jury can infer guilt (i.e., an innocent person would talk, so silence must establish guilt).